Why did my cat become so terrified of a cat we were babysitting that he hid behind my dog?

Well, maybe your cat just doesn’t work well with others. Most cats HATE having strange cats in their territory. When I got my new kitten, my older cat hissed at him even though the kitten was half his size and really scared. Also, we don’t see eveything that goes between cats. They use body language thta we don’t understand. The new cat could have been threatening your cat without you relizing.

I just dont know why.

Cats are very territorial. The new kitty was the invader, regardless that it wasn’t aggressive. You’re lucky your cat didn’t act out by “re-establishing” his/her territory. Your dog may have simply been protective of your cat, especially if they are “buddies”. The dog didn’t sense any iminent danger, therefore did not become agressive.
I wouldn’t recommend it again.

Whenever you bring a new cat into your home with an existing cat, it should be done slowly. You should never put them together immediately. It’s an invasion into their territory as the others said. It would have been better to put the cat you were babysitting into a room until your cat and dog had a chance to get used to the new cat’s smell and existance.

They managed to get along better after. I didnt just up and put em together. Thanks for the answer though. cause those first days im like “He’s afraid of something like half his size?”
Thanks for the answer.

maybe your cat is scared of other cats, i would keep him away from other cats and relax him by petting it.