Biri

A Link to the Past
These electrical jellyfish-like creatures are found mainly within the dungeons of Hyrule in A Link to the Past and come in two varieties: blue and red.

The weaker, blue Billy can be easily dispatched by attacking with a sword while it is not charged with electricity. The red Billy are almost identical, except that when successfully struck they will split into two miniature versions of themselves that must be dealt with similarly.

Ocarina of Time
Billy are small electrical jellyfish located inside Jabu Jabu's Belly in Ocarina of Time. They will electrocute Link if he comes into contact with them and can only be defeated with the Boomerang, Deku sticks, Bombs, throwing Princess Ruto at them, or by stunning them with a Deku Nut before moving in with the sword.

Oracle of Ages
Electrical jellyfish creatures appear in Oracle of Ages that may or may not be Billy; no official source is known to exist that determines the name of the creature.