Honestly the form matters less than the lifestyle implications. Pretty much any attractive form would get me off since I’m a degenerate. The main concern is lifestyle for us after the change. Since I like having a girlfriend and not a pet, I’d want something that would allow her to continue to live as a person. Something mythical could be exciting. A naga, sphinx, dragon, or even something along on the furry spectrum like a humanoid lizard or mammal.
I mean, I could just say a different human, but If you’ve got the opportunity to change into something fantastic you might as well go for it, know what I mean? plus I think it would be fun if she had a tail