
I just finished my hour of working on AWW, in which I worked on ways of doing animal companions. I'm going to go do the dishes after I post this, and then I'll be taking up the current (almost finished) chapter of Tales of MU, but first I wanted to talk about the results of this afternoon's work.
I had started with the idea of treating them as highly customizable Gear, but that didn't go very far... they were too complicated and too useful compared to anything else you could take in the same slot. They pretty much demand to be Archetypes. I'll probably keep Mount as an available type of gear, but if you want a really combat-useful mount, that's Archetypal.
In terms of dealing with specific types of animals or using generic ones and treating the differences among different animals as fluff, I split the difference between what I call "The Druid Approach" and "The Ranger Approach", from the way 4E handle's a Druid's beast forms and a Ranger's beast companions.
In 4E, the Druid has one generic "beast form" that is supposed to stretch to cover any realistic or fanciful animal of about the right size, whereas the Ranger has about 10 different categories (like cat, raptor, or lizard) to choose from.
In AWW, there are five different Archetypes suitable for animal companions: Clever, Fierce, Riding, Winged, and Wise. You get such a companion by spending one of your own Archetype slots for it. Much like similar companions in 4E, you spend your actions to command the creature during your turn in combat, but it also gets free actions based on the Archetype slot you give it: one move and one attack (or two moves) if it's your primary, one move or attack if it's your secondary, one move if it's your background. A free "attack action" isn't quite as unbalancing as it would be in D&D, for reasons that will become clear when I post the actual combat rules, and most animal companions are fairly ineffective fighters.
Clever Companions are small and nimble, able to do things that other companions can't like untying ropes, opening doors, and basically manipulating objects like they have hands (though they can't wield weapons). They're also natural climbers. Cats, foxes, rats, spiders, monkeys, spider monkeys and the like could all make a good Clever Companion, though no rule links the Archetype and these animals specifically. If you want a fox that mostly just bites people, it could be Fierce, or a cat who looks at kings and things like that could be Wise. Clever Companions are very weak fighters, but gain substantial bonuses when making non-damaging attacks (tripping up opponents, distracting them, etc.)
Fierce Companions are ones who are good at fighting. They're the equal of an armed fighter, and have a little more ability to fight independently. They have far less out-of-fight-scene utility, though they're physically the second strongest type of companion, behind Riding. Fierce Companions can be of just about any size from badger to bear.
Riding Companions are mounts. They're your size, or one size larger. Horses are the obvious choice, though anything from dogs and goats to fantasy creatures can be made to work. They're physically the strongest and second fastest, behind a Winged Companion that's flying. They're weak fighters, except when charging... and they can combine attacks and share certain bonuses with a rider when charging.
Winged Companions are flying creatures like birds and bats. They're normally small, like Clever Companions. They're not great fighters, but their mobility basically makes them into a living ranged attack. They are more alert/perceptive than most Companions, second only to Wise Companions.
Wise Companions have the greatest Perceptual Strength of any animal companions, and have more ability for their master/partner to profit from this. They also have a higher than normal Mental Strength, and can participate in any Mental Check the party needs to make as long as the hero they're attached to is there to translate for them. In combat they have no physical attack capabilities, but have the ability to use the Social Attack rules (to do things like stare down, intimidate, or calm enemies). A Wise Companion may be any animal that's stereotypically wise, but it could also be another type of animal that's magically gifted with intelligence, tied to a nature spirit, or is such a spirit embodied in flesh.
You can spend multiple Archetype slots to get multiple companions, but there's diminishing returns because of how the game's action economy (and actual economy) works. This is by design, as characters with hordes of built-in followers can bog things down, make the game less fun for everyone else, and can be unbalancing. As another perhaps more interesting option, you can take two or more animal companion Archetypes to represent a more powerful companion: Fierce Riding Companion for a warhorse, Wise Winged Companion for an owl or raven with stereotypical quasimystical attributes, Clever Winged Companion if you want the equivalent of a feathered universal remote so you never have to get up off the couch, or Winged Fierce Riding Companion for oh holy shit the Flying Bear Cavalry is here!
"Combo Companions" take the best abilities/bonuses of each of their components, and can be any size that's valid for any of the parts. A Pixie could take a Clever Riding Companion to have a squirrel or cat mount. A Human would have to stick with something larger.
All Companion Archetypes have access to a pool of Techniques (cf. D&D Powers)that represent animal adaptations... these Techniques are also used by Skinchangers (cf. Druid's "Wild Shape") for the special abilities they can gain. There's no distinction between having an Animal Adaptation Technique for your Animal Companion or yourself... if you're a Skinchanger with a Companion, you share the same ones.
Other Companion Archetypes exist and can be layered with these ones. They include Linked Companion, which focuses on shared senses/life force (like a wizard's familiar in some interpretations), Spirit Companion which is semi-intangible and can be banished and summoned, and things like that. There will probably be about ten different Companion Archetypes in total, allowing you to make a wizard's or witch's familiar, a knight/paladin's mount, a ranger or druid's animal companion, a "boy and his monster" type character, a faithful hound, a marquis's cat, a permanent undead servitor, or any number of other things.