I've never seen any cats hump anything, but my parent's dog is about 9 years old and has been neutered a long time and he still likes to try to hump things upon occasion. When I was home, he even tried to hump my daughter, who was lying on the floor! (mind you, i put a stop to that really fast!)
I've also seen quite a few geldings (castrated male horses) humping mares in their area. It seems to be a drive that just doestn't go away for some animals, despite the lack of proper "tools" to really get the job done!
Randi