I think this sentence would sound better as "Musens fiende er katten". (But "Musenes konge", the king of mice). As it is the singular form that is (mostly) used as the general species name, the group name, in Norwegian.
If you were in business with mice, or even better horses for the purpose of the example (since the plural and singular "mus" are the same), "å drive med hest". It can be the answer to "what do you like to do on your spare time". Or if you have been cruel to your animals, a verdict can be to not be allowed to keep horses or have responsibility of horses = "fradømt retten til å drive med hest".