If a wizard did it, I'd want to be able to say which wizard, and what their motivation was. Even 'madness' has motivation: paranoia, rage, homicidal pleasure, etc. (Of course none of these have much relation to real mental health problems.)
"Oh, that's an owl-bear. You'll find a lot of chimeric creatures around here, from when old Archmage Halitosus Flemfromage used to live up in the castle. He was experimenting with combining unrelated biologies. The fish-squids in the lake are edible, but watch out for spiderbats."
What a great example! With just the tiniest amount of detail, you can go ahead with all the silly, gonzo wizard nonsense you want and it serves to actually pull the players into the world rather than break verisimilitude. As a player, I really appreciate this sort of detail, and as a DM, I actually think it helps when creating an adventure. Magus Flemfromage there causes me to think of half a dozen ideas for interesting encounters that some generic wizard would not. I feel like there's this idea that adding details just makes the DM's job harder, but I honestly feel like it's usually the opposite.