Zylla was a lizard-like biped creature encountered by Miyamoto Usagi.
Two woodcutters discovered an enormous egg and took it to the hot springs to attempt to boil it for their dinner. However, the result of their actions caused the egg to hatch, and they fled when they saw Zylla inside it. At the same time, Miyamoto Usagi visited the hot springs to relax. However, a group of bandits saw Usagi carrying a large amount of money (half of a bounty for retrieving a statue of Kwannon with Murakami Gennosuke) and followed him to the hot springs, intending to kill him while he was defenseless and take the money. Zylla appeared and used his fire-like breath to incinerate the attackers. This led Usagi to wonder whether Zylla was in fact a God.
Zylla is clearly meant to be an infant version of the famous movie monster Godzilla. Usagi asks "Are you a God, Zylla?", and advises the creature to visit the big city when it grows up.
"Zylla" (collected in Book 2: Samurai).