1. The question is whether there is an issue of shatnez. Assuming the dead animals were kosher and slaughtered properly (e.g. you are mixing beef and mutton), that’s not a problem but it might be worth asking. However if you bred the animals together it might raise an issue, and with modern technology it might be possible to produce a sheep/cow chimera (i.e. a living cow/sheep mixture), which would be a new shailoh which will probably get asked some day, when its invented.
2. Fish would be a different issue.