The first fetish I've witnessed was a mother/son incest scene in a manga called "Please Miss Yuri" by Syowmaru. Caught me off guard at the time considering I was still fairly new to hentai, where a manga I stumbled upon concludes with a dude fucking his pregnant mother. It was by far odd, but highly arousing. Boner was definitely confused at the moment.
I also give shoutouts to the blonde chick from Bible Black Only who grew a cat o nine tails dick in order to fuck multiple girls. That was first exposure to futa.
Gravity cat wrote...
I first took a liking to catgirls because of Nina from
Words Worth.
Out of all the catgirl scenes I've witnessed, that one burned into my memory the hardest. Such a timeless hentai.