Lizard Egg

The odds are bad enough that nobody sets out to farm it. It either falls while you are clearing the temple for the traps and the altar, or it does not fall in that world at all. Flying Snakes drop it too, which is strange given what hatches out of it, and that is probably where the pet borrowed its wings from.

The lizard walks after you on the ground and only sprouts wings when it falls behind or you take to the air, at which point it flaps up to you like a flying snake and settles down again. That is the whole charm of it. Like every pet it costs you a vanity slot and does nothing in a fight.