hehe, i actually an idea similiar to this a while back, and i think i discussed it with Ketche(it could have been someone else, my memory's bad :/)
but my idea's a bit more complicated.
Creating a new petsystem not replacing the current one
- you'd have to go to a petstore and buy a pet(or maybe tame a monster or something, i'm not sure on how 'getting the pet' should work)
-there'd be a limit to the number of ppets you could have(1 would work)
-when using the command 'killpets, it won't kill this pet.
-the only way to kill the pet would either be A) give it to someone else(maybe have some sort of ownership card in inventory?) B) sell it back to the petshop (again, the owner card, and after the card's given away, the pet walks into it's cell or wherever, but if it's ownership card is just dropped on the ground, it'd just stay in that area until someone picks it up.) C) A monster kills it and permadeath is enabled(if it is not enabled, then you should be able to goto the petshop or maybe even a vet, and your monster will be there so you can pick it back up)
Experience:
-The pet would be able to gain exp the same ways you can(well, from killing monsters, and possibly identifying items(?))
-It'd basicly share exp as if you were in a party with someone
-when you first get the pet, it'd be a puppy(if it's a dog) and the title could change as it levels up
the pet:
-there could be a whole new set of items/weapons/armour made for pets, that can be bought at a petshop. For instance, you could buy a collar which could act as a container and you could put an inscribed scroll inside it with who the pet originaly belongs to, etc. or maybe, you could buy a saddle and be able to ride the pet for an improved speed and faster travel across the world map(i dont know if you'd still be able to attack while riding, realisticly you could, but it could lead to abuse). maybe different armour they can put over their paws/feet/hands to improve attack or something.
-Weapons: a four-legged animal wouldn't really be able to weild a sword or bow or the like. they'd have their fangs and their claws as natural weapons. a dagger could be a possibity(a dog could hold a dagger by the handle and jump to slash somethin, right?

) something would have to be worked out to give pets their own unique weapontypes
-Spells: the pet would be able to learn new spells, but they would have to be 'taught' how to cast a certain spell. So you could A.) Cast a spell often enough and kill lots of monsters, and after a while the pet will learn it from it being done(this may not be the best way to achieve this). B) take your pet to the petshop, or vet, or maybe a pet school. pay a small fee, and place a spell book on an altar, and then a trainer will take your pet into a backroom or something, and give you a ticket and tell you when you should come back to pick it up(anywhere from a few minutes to a few days, depending on the spell and pet's exp and Int). When you come back, if the pet failed to learn the spell for any reason, the shopkeeper/trainer could give you a refund.