I have to say, I have read a ton of isekai novels. Mostly they are shit, but there are few gems out there, these gems are mostly pushed into the mainstream, so you might know them already.
But I'm not here to just speak about isekai shit, I want to talk about being isekai into an animal. I have read a few novels with this premise. 5 of them to be exact. 1 of then off-site.
Here's the ones you can find on Webnovel. Lion, stingray, ant, and a cat. Chose, if you want to know the titles, ask in the comments. I don't quite remember the titles.
P.S. these are the ones I know, if you know some more, let me know.