Answer:
'She' is a cat.
Step-by-step explanation:
Clues to this answer:
"She stretched her legs, one at a time." It clearly isn't a snake, they are legless. It can't be a bird, they sleep in trees/nests. Not a girl, as it isn't a human child sleeping outside.
"Who knows, maybe I could catch a mouse!" Using the conclusions from the other clue, this is defiantly a cat. Cats love mice, they have four legs, and they'd happily sleep outside.
So, your answer is: C: A cat.