Can we agree that if everyone in the party has a pet monster of roughly equivalent power that they are all roughly balanced?
Sure, but that assumption is setting up a situation wherein, for whatever reason, one player
can't get a mount,
won't or simply, isn't interested in the additional management (raises hand). I
like the Ranger as a class, thematically, in most editions as well as the Druid, but I've had no interest in animal companions since I basically started playing D&D. Even familiars sort of put me off. As @
I'm A Banana points out, these animals can, on their own, do almost double the damage of players on their own. Unless completely rebuilt "mount" versions of these creatures are going to get made up that bring them in line with the player, but that still leaves you with a player basically gaining a DPR clone of themselves. Do we tell the guy who isn't interested in investing in Ride or Handle Animal, "So sorry, tough luck?" What do we do for the guy who just doesn't
want to deal with it? "Tough that's how this game works?"
The problem then becomes, Bob, riding his trusty steed Seabiscut, will be shelling out double his usual DPR, which means we need to up the ante of your average enounter but +/-DOUBLE, especially if becomes 2/4, 3/4 or 4/5 party members. Three Kobolds becomes 6. 6 Kobolds becomes 12. What happens when the Boss Kobold makes a call that the lunatic on the bear is too tough for them and that they ought go poke Bill over there, who has nothing but his pointy sword to protect him? Suddenly Bill is in a really dangerous situation simply because Bill didn't want to hassle with the work required to earn, maintain and manage an animal companion.
I mean I agree getting to ride a Battle Bear into combat is fricken awesome. But perhaps it would be better for these to be special situational mounts, for attacking other enemies of the mount's organization, who themselves ride Battle Apes? Players would then not be required to manage a companion 100% of the time, players who don't feel like grinding rep would not be missing out on something for a
majority of the game, and it would only require the DM to design specific fights to include these Battle Mounts, instead of upping the ante on ALL of them.
EDIT: there should also be some kind of
downside to riding such a beast. A social one where you cannot befriend certain organizations anymore, or maybe they just can't fit inside your average dungeon crawl.