Yea, been trying to find one for years. Closest I had come to was Rappelz. They leveled alongside you, had thier own skills that you picked and chose with the points they gained per level. Multiple pets with a variety of types who's builds could be picked and chosen by the player for versitilty. Sure there were favorite builds but playing it I made my own. I blew some minds when my Poultry kept stealing hate but had enough toughness in the build I had to tank successfully. Then.. rappelz fell to the dark side.

Now you can make them stronger, faster, with exotic colors and all you have ot do is butcher other pets that were 'tamed' as in accepting your desire have you fight with them only to have it's life cut short, betrayed to give your chosen pet a slight edge in power. Ugh. Havn't been back since, sick of those types.

Where can I find a pet system that has active pets with diverse skills, purpetual presence in the world, creates a solid impact in combat so you actually want the pet alive and kicking at your side. And absolutely not a butcher fest or cardboard cut and paste of one of the 4-5 main types of pet systems you can find in almost any game.

I've played most of the mainstream mmo's, quite a few of the small name'd barely known. I'm sure there are others, if you know of it, please point the way for me.

Sincerrely,

      The pet Fiend.

Have a look at Wurm if you never played it. 

Are you looking for pet taming or pet summoning? Many MMOs have at least pet summoning. Personally, I love pet taming, although, Summoning isn't bad either.

Game's I've played with pet systems. 

Rappelz. Highly flawed due to RNG cash shop lottery ties, but if the system was to be incorporated into a different kind of MMO, It'd be the best pet system ever implemented in an MMORPG.

WoW. Not a bad pet system. Broad choice of pets to tame with different roles and functions. I have always enjoyed the Hunter class.

Rift. Every class has a pet option. They aren't tamed, they are summoned and standard for the class, but the pet system otherwise functions just like what you'd find in WoW. 

Anarchy Online: Very unique approach. Bureaucrat: One of the best pet classes in any MMO ever. You have one permanent summoned pet. then you have 2 charms. One is a long term charm another is a short duration charm. When the charms expire, you have to re chamr them or you end up with an even bigger problem. LOL. The Metaphysicist uses 3 summoned pets. One does CC, one does healing and one does DPS. Really fun. Then the Engineer uses summoned Bots for pets they can tank and DPS but it's tricky because they don't hold agro as well as you'd like.

Perfect World. Think Hunter from WoW. Very similar in overall playstyle.

GW2: ranger class. Again, Tame many pets from the game for custom playstyles.

I could probably dig up others but those came to mind 1st