Here are some stats that I think should be included if the combat system is going to be put in place.
Health
Happiness
Hunger
Attack
Defense
Stamina/Magic
I think Hunger and Happiness should be very basic and users shouldn't have to worry too much about these.
I think that your pets should faint instead of die so that it's easier on the user who owns them.
Hope to see these things pretty soon, and be sure to make the pets as basic as possible