Just for giggles I made a level 20 champion mastiff so I could spitball what kind of power Biscuit would wield.
That thing is a true beast, hard as nails to take down. Especially if it is mounted by a halfling Paladin or Cavalier, who could prolong it's longevity. Though a (dire)wolf or giant spider would fair even better on the offensive side of things.
But yeah, these things aren't "sidekicks" they are "substitute PC's." They even outright rival some of more questionable subclasses without picking a good base (especially the Spellcaster, who would make a Sorcerer or Warlock think twice about their life decisions). I don't care for how the Expert is set up, because the Expert doesn't have a proper way to scale attacks. However even that can be mitigated by picking something that already has multi-attack. Just imagine a Deinonychus Expert, clever girl indeed. The gap between a good base creature and a poor one is the most noticeable there.
In the end, I can't help but compare these to the Retainers from "Strongholds and Followers". Mostly because that book just got it's PDF out. While I do like having ultra-durable mounts that can actually take a Fireball or two to the face, I do think that the Retainers have a better power curve for an NPC that is supposed to support the PC's instead of stand alongside them as an (near) equal.
