Answer:
A spent round or a slimy critter.
Creeping on its belly, curled into a ring. Eyes forever open, beware its lethal sting
It has plenty of backbone but doesn't have a let. It peels like an orange but it comes from an egg.
It can be white or blue or black, Humped or have a horn. And when it talks it sings its menuets, as it's travelling the depths.
What does a cat have that no other animal has?
My days are numbered.