Wolf Link

Wolf Link is the sacred blue-eyed beast in Twilight Princess. He is the "divine beast" that Midna explains the Twili have long believed would free them. Link is transformed into this Wolf form due to the influence of the ever expanding Twilight covering Hyrule. His whole body changes and reforms and leaving only his earrings and distinctive blue eyes. Though losing the ability to use items, the new form grants Link heightened senses and speed, along with many other unique abilities. In this form Midna rides on Link's back giving directions, and allows him to warp using the Twilight Portals created across Hyrule.

Transformation
Link is first transformed into the Wolf following the appearance of Bulblins at Ordon Spring. Colin and Ilia are carried off, and Link is left lying unconscious in the Spring. When he awakes, Link runs in pursuit of his kidnapped friends, but finds a barrier of Twilight blocking his path. Suddenly, the massive arm of a Shadow Beast springs from the Twilight, dragging Link inside. As the Shadow Beast clutches Link around the neck, the mark of the Triforce suddenly appears on the back of his hand and bursts forth pure light, driving the creature back. Link falls to his hands and knees and morphs from human to wolf, causing him to pass out from the strain of the transformation. He is then dragged off by the Shadow Beasts and imprisoned within the sewers of Hyrule Castle, where he will meet Midna who helps him make his escape.

It is later revealed that anyone engulfed in the Twilight will normally be transformed into a helpless Spirit, and that only the power of the Triforce protects Link and enables him to retain his physical form, although considerably altered. Link will eventually be able to return to his human form with the aid of the Light Spirit Faron. Upon entering the other provinces of Hyrule covered in Twilight, Link will again be transformed into Wolf form, being able to once more regain his human form after aiding the Spirits of Light by restoring light to the area.

A significant turning point in terms of the transformation comes when Link finds the Master Sword and receives the Shadow Crystal. With this artifact, he is able to change at will by exposing himself to the power of Twilight contained within the crystal by simply talking to Midna.

Midna
Following Wolf Link's initial encounter with Midna in Hyrule Castle sewers, she will always ride on his back while in Wolf form, except while in serious combat (or when warped out of the Twilight in the early part of the game). When he reverts to human form, Midna dismounts and hides within Link's shadow. Interestingly, at the beginning of their relationship Midna treats Link almost as a slave, riding imperiously on his back and giving orders. The relationship gradually changes, a pivotal point being the aftermath of Zant's attack which leaves Midna helpless and mortally wounded. In a touching sequence, Wolf Link carries the barely conscious Midna slumped on his back, to seek help from Princess Zelda.

Senses
One of the most important aspects of Link's wolf transformation is that he gains the use of greatly enhanced senses. This ability, shown as "Sense" while in Wolf form, activates a special combination of enhanced visual and olfactory perception. Visual range is reduced, but short-range Link is able to detect many otherwise hidden things. In gameplay terms, the Sense ability is notably similar to the power of the Lens of Truth from previous titles, though it can be used indefinitely and does not take up magic power (as there is no Magic Meter in this game.)

Wolf Link's enhanced Senses have a number of vital applications throughout the game. The first use, taught by Midna while in Hyrule Castle sewers, is to detect the Spirit forms of the people of Hyrule trapped within the Twilight. Appearing as a floating greenish flame, these Spirits' true form can be seen by activating Sense, enabling Link to listen to their thoughts. As Link is unable to communicate directly with the Spirits, this enables him to gain useful information from them about recent events. Also, Link needs to use Senses to see and defeat certain enemies, such as Ghoul Rats and Poes.

Another application of the Sense ability is the detection of special patches of ground found all across Hyrule. These appear as patches of sparkling light where Wolf Link can Dig, usually unearthing items such as Rupees or Recovery Hearts. Some of the patches are larger, and can lead to different areas within dungeons, or to caves. The Dig ability is always available while in Wolf form, but (with the exception of a few instances) is useless unless combined with Sense.

Scents
A further, important use of Wolf Link's enhanced Senses is the detection of special "Scents" encountered during the quest. Upon encountering particular items, Link is able to remember the Scent, which appears as a colored trail while Sense is active. This tracking ability is vital at many points in order to progress the story. When a new scent is encountered, the previous scent will be forgotten. The scents are as follows:
 * Youth's Scent
 * Ilia's Scent
 * Poe's Scent
 * Reekfish Scent
 * Medicine Scent

Midna's Assistance
While mounted on Wolf Link's back, Midna can use her powers to help Link in a number of ways. In certain situations, she is able to form her hair into a giant, orange (in light) or pink (in Twilight) hand; for example in Hyrule Castle sewers, she will use this "arm" to draw Link's attention to chains which must be pulled to open grates, raising and lowering the water levels. One particularly noteworthy use of this power is during the battle with Dark Beast Ganon, where Midna is able to grapple the enormous beast to the ground as he charges, allowing Link to attack. She also uses this arm to open dungeon doors for Link (in human form, Link does this himself).

Perhaps the most powerful ability granted to Wolf Link by Midna is the dark energy attack. In this technique, Midna raises her hand and summons a growing circle of crackling Twilight energy which will surround any enemies within range. Once released, this power allows Wolf Link to unleash multiple attacks, rapidly darting from one enemy to another, defeating them all simultaneously. This technique is vital to defeat the groups of Shadow Beasts which appear from the Portals across Hyrule. It is also very useful in defeating large groups of enemies.

Another extremely useful ability which Midna is able to extend to Wolf Link is the power to Warp to any of the Twilight Portals across Hyrule. This ability proves invaluable in enabling Link to traverse Hyrule, in combination with the ability to control the Wolf transformation with the Shadow Crystal.

Attacks
In accordance with Link's new Wolf form, the nature of his attacks in this form is also different. The standard Bite Attack is performed in the same way as a sword slice (in the Wii version by lightly moving the remote from side to side) and has a similar effect. The Jump Attack is also performed similarly to when in human form, however against certain enemies Wolf Link can repeatedly bite at an enemy's jugular by repeated button presses. Also available is the Spin Attack, again performed in the same manner (by shaking the nunchuk). Once completed, Wolf Link's tail will glint in the same manner as Link's sword, indicating that the Spin Attack can be used again. The most powerful form of attack is the dark energy attack detailed above.

Other Abilities
In addition to the abilities shown above, Wolf Link also possesses a number of other miscellaneous special abilities. While running, he is able to dash; pressing the A button will allow him to briefly run more quickly, causing Midna to gasp and hold on tight.

In various locations across Hyrule, Link will encounter tightropes which must be crosses to reach other areas. Wolf Link is able to rope walk by balancing carefully and walking slowly across.

In other specific areas without ropes, Link is able to use a long-distance jump. These areas are indicated by Midna, who will then float across to show Link where to find his next foothold. This can extend into a chain of up to a dozen jumps which must be performed in rapid succession in order to reach the other side, sometimes avoiding enemies or falling objects.

Another useful ability is that of communicating with animals. In Wolf form, Link is able to understand and speak to all types of animals found across Hyrule, including Epona. He will be able to glean useful information and hints from the animals.

Being a Wolf, Link in his transformed state is able to howl, which can again be done is specific locations. Patches of Hawk Grass and Horseshoe Grass can be used to howl the tune normally played, summoning a Hawk and Epona respectively. More importantly, special Howling Stones can be used to summon the Golden wolf, enabling Link to learn the seven Hidden Skills.

Trivia

 * Some believe that how Link in A Link to the Past turns into a bunny upon entering the dark realm, reflecting his nature, that the Link of Twilight Princess may also be reflecting his nature when he enters twilight and transforms into a wolf. As a side note, in the A Link to the Past comic by Shotaro Ishinomori and in the A Link to the Past manga by Akira Himekawa, Link is shown to begin a transformation into a wolf-like creature in the Dark World, before he reverts to human form.
 * The colors of Wolf Link's fur are reminiscent of those of a grey Wolfos.