2Dropped by
getting one
Only Dark Mages drop it, and they are the summoner enemy that appears partway through the Old One's Army. Farm the first tier rather than the third: the odds are better there, the waves are easy, and you can run it back to back with a cheap crystal instead of committing to a full late game defense.
The pet is a cat with a propeller strapped to its head that drifts along behind you. No buff, no light, purely company.
the name
Gato is Spanish and Portuguese for cat, so the item is literally called Cat Egg. The design comes straight from Dungeon Defenders 2, which is the game this whole event was built as a crossover with, and the propeller cat is one of its pets.