Years ago we got a kitten and then a week later we got a puppy. Both were about 8 weeks old, both were male. The puppy was a big fella, but he had been born into a house where there were multiple cats, so he was familiar with them. He saw the kitten as his new best friend and wanted to play with him all the time.

At first the kitten was scared of the puppy. But pretty quickly he realized the pup was friendly. And then they developed some really bizarre games together. Like the cat would stick his head into the dog's mouth and let him slobber all over his head, then he'd pull his head out and hang by his front claws from the dogs jowels. They were both fine with this.

Eventually they started doing their own things and played less together, which you'd expect. But occasionally they'd rekindle their weird game sessions.

I didn't think it was detrimental to either one. But it probably varies wildly based on the temperament of both pets involved.