I sent a pet to a friend who is a returning player using the Pet Transfer option. She sent one back to me. I want to accept it, but the only option I have is to Dismiss. Why is that? Do I have to wait until tomorrow to accept it?

did she already accept the pet you sent her? the only reason i can think it wouldn't work is if your pet slots are full; can you check what it says on your pets page? maybe there's a period of time after transferring a pet during which the pet slot is still considered in use.

Yup She already accepted it. I have 40/41 slots used. I can only guess there is a cooldown time...