Li'l Devil

Mad Batter is a creature seen in and. While his name is never mentioned in game, in The Legend of Zelda - Link's Awakening Player's Guide, he is referred to as the Mad Batter. As the Mad Batter's name implies, this creature appears to be a bat. He sleeps in altars found in underground caves, and Link may awaken him by sprinkling Magic Powder on the said alters. The bat creature is known for providing Link with upgrades to punish him for having been released, but he inadvertently helps Link out.

A Link to the Past
In A Link to the Past, his altar is found in a hole next to the Dwarven Swordsmiths' Smithery. The hole can only be reached by pounding the stake blocking the way with the Magic Hammer and jumping off a cliff into it. Once inside, Link can go north to an altar and sprinkle the Magic Powder. An angry bat will appear, and scold and thank Link for waking him. This confusing shift in his dialogue continues until he finally cast a spell on Link, halving his Magic Meter. In reality, this causes Link to consume half the magic power he normally would with pertaining items.

Link's Awakening
The "Mad Batter" shows up in three different locations throughout Link's Awakening: Mysterious Woods, Martha's Bay, and Tal Tal Heights. His altars are always found underground. The Mad Batter serves the same purpose wherever he is seen: to increase Link's inventory. This time, his altars look like lamps. When Link tosses Magic Powder on them, the creature appears in a ball of light form before taking a bat-like form. Upset about being awakened from his sleep, the Mad Batter speaks with the same mannerisms as he did in A Link to the Past. The creature will triple Link's capacity for carrying Bombs and Arrows and double that of his Magic Powder,  thinking he's troubling the hero by burdening him with more equipment. This time, Link has a choice of whether the creature "punishes" him or not, as the creature will ask "Are you ready?!" before he casts a spell. However, if Link answers "N-no", the Mad Batter will only change the option of what equipment he will increase.

Trivia

 * The Mad Batter's name is a possible reference to the Mad Hatter in Alice in Wonderland.