This is a decently fun game with a lot of potential, though a big issue I had is managing to select the rats. They run around so much that it's pretty hard to grab the right one, and then drop it on the correct other rat, even with just 5 or so on screen. Maybe a selection button would work better? Like click two and it highlights them, and click a separate 'confirm' button to breed? It'd also stop the accidental breeding I kept having when just trying to move the rats around c':
Aside from that though, this was pretty fun! It was interesting to see the ways the rats developed, and I enjoyed the spontaneous mutations that sometimes popped up :D I also liked that you'd sometimes get new creatures wandering in to spice things up c: