No judgement please 😔

But I'm looking for a manga with animal transformation, where the seme [accidentally] transforms into an animal during sex

Hide and Seek, 9th story, Loup-Garou Gokou, somehow fits.

Tsuki ni Ookami : They don't transform during sex but they do have animal ears....

well, i remember there was this one collection of one shot. it contained a story where (was it uke or seme) that transformed into a hamster when he woke up. the other partner got exited and they had sex. (they were rising a hamster, and they thought that maybe the hamster's power got transferred or something)

the series included other one shot where the uke pretended to be a dog (wore a tail strap) and seduced the seme (who is non-gay) and the story where two lovers were in need of some spice, so one of them dressed up in a bunny costume. the other partner laughed and said "so that's what you've been up to"

http://www.mangaupdates.com/series.html?category=animal%2Btransformation&genre=Yaoi&perpage=50
recommended,

Suzu no Ne ga Kikoeru, but no transformation, just ears

The mangas Hespia are talking about are by Sakira