I play Mweor, which is a fantasy cat-breeding sim. Here's the link:
http://mweor.com/I'm Squirrelypaws there, player #764. If anyone from the Arena joins, please feel free to message me; I'd be happy to help you get started.
Sadly, Mweor's owners announced a few months ago that they were burnt out and wouldn't be updating anymore, but honestly, I still highly recommend it. There's enough variety in Mweor breeds and markings already in place where you can keep things fresh for yourself, even if there's nothing new coming out.
I also play Lioden, which is a more realistic lion breeding sim. I truthfully find Lioden to be very tedious to play, so I'm not super active there... But it's still an interesting game. The link to that is:
http://www.lioden.com/