Do not feed or water birds and animals that can forage for their own food – it is an unnecessary tircha. This is true even if they belong to you. (ShA 11 – MB 29)
Hogs and dangerous dogs are not allowed to be raised. Therefore feeding them on Shabbos is not allowed. (MB 31)
Animals and birds that depend on you for food may be fed on Shabbos. (ShA 11)
A dog that does not live in your house is allowed to be fed on Shabbos. There is a mitzva to have special compassion to feed dogs. (MB 31)
It is not proper to feed the birds on Shabbos Shira. (MB 31)
Do not put the leftover dog food in the bowl of the cat (This is true of any 2 distinct species. Dogs and cats are examples.). The cat might find the slobber of the dog repulsive and refuse to eat it. It will come out to be an unnecessary tircha. You do not need to protest against someone who is lenient in this. (Rama 14 – MB 38)