I had this happen once. I started my family of three with a horse in CAS, but when moving, the horse somehow got stuck on the porch. I couldn't get him off - he couldn't use the stairs. I tried everything...deleting the stairs, using every cheat
but the one that would've helped...In the end, I remembered that social workers could magically transport children to their car, so maybe it worked the same with pets?
I wound up putting Charms the horse up for adoption, and, as the social worker spirited him away, I realized that, straight through the house, was another door leading out back. Man, did I feel stupid!
I wish I'd known about being able to adopt pets back...
I felt like crying, especially since my sim looked so sad.