Chuchu

ChuChus are jelly-like monsters that appear in many Zelda games beginning with Majora's Mask. They are likely relatives of other jelly-like enemies from earlier Zelda titles such as the Buzz Blob and Zol. The ChuChu comes in a wide variety of colors and types, including Red, Green, Blue, Yellow, Purple/Dark, Rock and Spiny. As an enemy, they are relatively simple to beat; a simple slash with the sword is all that is required to take down most varieties.

Characteristics
ChuChus, though they have a standard appearance in every applicable title except for Majora's Mask, the general behavior and physicality remains quite constant despite differences in appearance.

In Majora's Mask, ChuChus appear as bottom-heavy blobs of transparent jelly in either red, green, yellow, or blue, with a large, grinning mouth full of crooked teeth and round eyes on stalks. They move around normally by slowly oozing along the ground, but once they see Link, they attack by hopping towards him in an attempt to knock him down. However, they can be easily defeated with one or two hits with a sword. Dependent on color, they contain various items within their bodies, such as a Recovery Heart (red), Magic Jar (green), or Arrows (yellow). Blue Chuchus have no items inside their bellies, and are used only as stepping stones when frozen in a block of ice via Ice Arrow.

In Wind Waker and several other titles afterwards, they take on a much different appearance: they are top-heavy blobs of opaque jelly with larger eyes that are no longer on stalks, and move about either with the pseudopod-like roots at their base, or by hopping around. They also come in a new color; purple. When left alone, they can shrink down into a tiny, slow-moving puddle.

As the later titles progressed, different types of ChuChus emerged with simplistic, yet highly varied personalities including the incorporation of electricity, rock-like helmets, and ice powers into their battle strategies. Nevertheless, the ChuChus kept to their Majora's Mask roots for the most part, as even into Spirit Tracks they used color to denote behavior, power capability, and in-game frequency. Of the ChuChus in the Zelda universe, the red variety is the most common, however, the green, yellow, and purple versions have seen quite the incorporation as the series progressed through the years.

In Twilight Princess, the Chuchus are called simply Chus, and are little more than large, amoeba-like blobs of opaque slime. While they behave similarly to other ChuChus in that they denote characteristics and type by color,  move slowly, and tend to jump on Link when threatened, these Chus also have the ability to split and merge. A large Chu will split into two smaller Chus when attacked, and will keep splitting until they are too small to do so. When two Chus of different colors collide, they will merge and grow, retaining the color of more common Chu (from most to least: Purple, Yellow, Red, Blue, Rare).

In few titles, such as The Wind Waker and Twilight Princess, Chus tend to leave behind Chu Jelly, a vital ingredient used to synthesize potions at the local Potion Shop. A certain number of Chu Jelly ingredients are required to help the potion chemist at the Potion Shop make a specific potion; the color of the ChuChu denotes the type of potion its Chu Jelly can be made into, for example, a Green Chu will yield Green Chu Jelly good for making a Magic Potion (as it is also green), once the required quantity of such Chu Jelly is acquired.

In Skyward Sword, they are referred to as Chu-chus.

Red ChuChu
The Red ChuChu is the most common of all versions, and thus is the face of the ChuChu in terms of standard behavior and  appearance. Their greatest example is seen in The Wind Waker; they are the lowest form of ChuChu and the easiest to slay. Their attack deals little damage, and can be tricky in groups (though rarely forming any more then three at a time). In Majora's Mask, the Red ChuChu releases a Recovery Heart when killed, but in later titles such as Twilight Princess, the creature yields Red Chu Jelly, which in turn can be drunk on the  spot to refill eight hearts and boost attack power or can be used to  synthesize a Red Potion in The Wind  Waker, one that specializes in life replenishment, at a Potion Shop.

Green ChuChu
The Green ChuChu appears legitimately in four titles, and has a drastically different appearance and behavior in each appearance. In Majora's Mask, the 'Green ChuChu contains a Magic Jar which will be dropped once defeated, otherwise it behaves like and is often apparent in the same area as it's relative, the Red ChuChu. In The Wind Waker, they will drop Green Chu Jelly when defeated, which will restore magic power. They also appeared in The Minish Cap, and in Phantom Hourglass, Green ChuChus only appear in the Temple of Courage. They have the ability to shrink into a puddle to evade Link's attacks. Other than that, though, they can still be easily defeated with a swipe of the sword.

Interestingly, Green Chus are extremely rare in Twilight Princess, and the Chu Jelly they provide has no effect on Link whatsoever, leading theorists to believe this Chu was created for the sole purpose of replenishing Link's magic meter, seen only in the beta version. Since the magic meter in Twilight Princess was removed, perhaps the usability of this Chu's jelly was removed as well.

Blue ChuChu
Blue ChuChus, pale-blue in color, are one of the rarest types, found in small, always significant quantities in the titles they appear in. In Majora's Mask, the Blue Chuchu holds no items, and is described by Tatl  as "basically just a blob of water". Only a single example of this color ChuChu appears in the game, found in Great Bay Temple where it must be frozen using an Ice Arrow in order for Link to reach a particular area. Blue ChuChus are also the only type of ChuChu that can be frozen in Majora's Mask.

The Blue ChuChu in The Wind Waker, The Minish Cap, and Phantom Hourglass is highly electrified. It also drops the rarest type of Chu Jelly, Blue Chu Jelly. The Blue Chu Jelly fully replenishes both Magic Power and Recovery Hearts, once made into a Blue Potion by Doc Bandam, or when drunken immediately from the bottle after defeating it in Twilight Princess. To emphasize their scarcity, there are only twenty-three  Blue ChuChus on various islands in The Wind Waker, only dropping their rare Chu Jelly once.

Yellow ChuChu
The Yellow ChuChu is unique in that it is electrified in all of its incarnations save for its appearance in Majora's Mask. In Majora's Mask, the Yellow ChuChu holds an item, usually Arrows, and appears in only a few locations (usually within dungeons). Yellow ChuChus are even rarer in The Wind Waker than the already-uncommon Blue ChuChus. These foes can only be struck with the sword in close combat when their protective energy field is inactive. Link must either wait for their electricity to fade or stun them with an item such as the boomerang or the Whirlwind in order to inflict damage. Bombs are also effective to use to stun a Yellow ChuChu, or even to penetrate their electric defense.

Interestingly, it does not drop an individual Chu Jelly in The Wind Waker, instead rewarding Link with either Red or Green Chu Jelly upon defeat. When destroyed, and the Chu Jelly received from this foe in Twilight Princess can be used in the same vein as Lantern Oil.

Purple ChuChu
In The Wind Waker, Purple ChuChus are only harder to find than Blue ChuChus because they tend to live in areas away from direct sunlight, within the ancient temples. Purple ChuChus are considered the most impregnable, as almost any attack against them is useless. When a Purple ChuChu is attacked with the sword, bow or even the Skull Hammer, they will break apart and reform; the only known way to beat Purple ChuChus is to force them into direct sunlight, which will turn them to a breakable stone. Like most other enemies, Link can shoot it with a Light Arrow to destroy it, as well.

The solid Purple ChuChus of Spirit Tracks give off an an electric aura, but unlike Yellow ChuChus, they cannot be stunned. The most creative way to defeat them is to strike them with a sharp object attached to the Whip, such as the helmet of a Helmet Chuchu (with a metal, spiky helmet this time). A Phantom's blade, arrows or bombs can also defeat these. When the Chu Jelly of the Purple Chu is drunken in Twilight Princess, it will yield a random effect similar to the Nasty Soup, sometimes positive (i.e. replenishing 8 hearts) or negative (i.e. taking away hearts).

Big ChuChu
A Big Green ChuChu is the boss for the first dungeon (Deepwood Shrine), in The Minish Cap. It is really just a normal ChuChu, but seems enormous because Link is Minish-sized at the time. A Big Blue ChuChu is a miniboss in the Temple of Droplets, which is Minish-only as well. Unlike Big Green ChuChu, Big Blue ChuChu can electrocute Link.

Helmet, Rock, and Spiny ChuChu
These three ChuChu varieties are very similar as they all involve the encasement of the head of the ChuChu with a hard surface, whether it be rock or spikes. Helmet ChuChus, as they are called in Spirit Tracks, pay tribute to their eponymous name with the spiky helmet covering their entire heads. The helmet must be pulled off using the Whip, at which point the helmet can either be thrown back at the ChuChu to defeat it or thrown away to finish it off with the sword. The Rock ChuChu reverts to a Red ChuChu after its hard helmet is destroyed with a sharp blow to the head, in The  Minish Cap, or destroyed with a Bomb, pulled off with the Grappling Hook or smashed with the Hammer in Phantom Hourglass. In the latter game, this ChuChu variety likes to hide out beneath large rocks, busting out whenever Link draws near. The Spiny ChuChu has an excellent defense mechanism, puffing out spikes all over its body when approached, in The Minish Cap. To defeat them, Link must wait for them to retract their spikes, or use  a bomb to destroy the creature immediately.

Ice ChuChu
The white Ice ChuChus of the Snow Realm in Spirit Tracks need to be stunned using the Whirlwind or the Boomerang before attacking. Otherwise, they freeze Link, requiring the player to rub the screen with the stylus to thaw out the young hero.

Rare ChuChu
In Twilight Princess, Rare Chus are pearl-colored and sparkle. Rare Chus can be found in any area Chus are normally found, however the chances of finding them are incredibly slim. The Cave of Ordeals and three secret grottos, under a tree at Snowpeak, and in the Gerudo Desert are the only certain places to find them. Outside the cave, the small ledge on Death Mountain, under the large tree atop Lake Hylia between midnight and dawn, and also near Snowpeak are places reported to have had Rare Chus. A Rare Chu can also be found within a secret grotto beneath Ordon Ranch, accessed via a dig spot within the barn.

Rare Chu Jelly acts the same as Great Fairy's Tears. However, scooping up Rare Chu Jelly is difficult, as not only are they hard to find, but the Rare Chu is the first to be targeted by Purple Chus to merge. If a Rare Chu and any other Chu merge, the Chu will become the other color, canceling out the Rare Chu. Additionally, if you get Rare Chu Jelly, any Rare Chus set to appear will be replaced with Yellow Chus.

Trivia

 * If a Blue Chu and Yellow Chu merge, they will create a Green Chu in the Wii version of Twilight Princess. There are few cases of these two types being in the same area, so it is very hard to create a Green Chu. The best place may be the Cave of Ordeals on Floor 12. To perform Link must have some Rare Chu Jelly so that the Rare Chu in the room is replaced with a Yellow Chu. The young hero must tread around carefully to avoid any Purple Chus or Red Chus fusing with the Blue or Yellow Chu. The resulting jelly from a Green Chu does nothing, and is likely a remnant from an earlier version of the game that featured a magic meter. Attempting to fuse a Yellow and Blue Chu in the GameCube version will result in a Purple Chu, but Green Chu Jelly can still be obtained using a cheating device. Additionally, one can obtain Black Chu Jelly with a cheating device, which has the same effect as Nasty Soup (random effect).
 * ChuChus in The Wind Waker bear somewhat of a resemblance to the Pokémon Wobbuffet from the Pokémon series.