I just brought my cat home from the vet an hour ago after being gone all day having dental surgery when he came out of the crate my other cat growled and hissed at him I don't understand why
Hi!
He smells different from bring at the vet.
Try taking a soft cloth, gently rubbing it on the cat that stayed home, and then rub it on the other cat to transfer familiar scent to the cat that went to the vet.
That is very common, and happens often when one cat returns from the vet smelling differently. It's called non-recognition aggression. TCS has an article with more info: How To Deal With Non-recognition Aggression In Cats
Very normal.
Watch the cat that had surgery but we've experienced this a lot. Cat comes home,smells different, acts different, the other cat(s) freak out and it's just one holy mess for a few days.
If it gets bad, just do as the other said and wipe him down, and keep him quiet (probably a good idea since he just had surgery anyway) in a separate room until he feels his best and gives himself a nice bath or two.
One of our cats spent the day at the vet, and when he got home, his litter mate best friend ran up to see him, froze, hissed and ran off. He acted hurt.
The next day, he acted less hurt. The day after that, he thought it was a game and playfully chased her.
They got along just fine after a few days. Don't worry unless it goes on for too long and the cat that had surgery isn't acting right.