The same rules apply on itch.io as anywhere else. Fan games are in a legal gray area, but we don't care unless the owner of the characters issues a DMCA request. Likewise for any other franchise. "Royalty free" means you don't have to pay per copy sold, but if in doubt ask the creator for details. If you don't know the origin, you probably don't know how it's licensed either, so you have to assume it's all-rights-reserved. Stay away.
Hope this helps, and ask if you have more questions. This is a big complicated issue, and frankly a burden on creators. I'm sure there are big FAQ lists out there with more detailed answers, but couldn't say where. It's just stuff you pick up over time.