The problem with basing it off Proficiency bonus means that you're basically tying mounts to character level. And that doesn't make a huge amount of sense. I shouldn't need to be a 10th level character to ride an elephant, for example.I suppose that you could tie the action economy together. Controlling a monstrous mount in combat, for example, might tie up your concentration "slot". Getting hit and losing concentration might cause the mount to act erratically until you can regain control (which is where your handle animal checks come in). Considering that monstrous mounts might just eat the rider from time to time, this could have some serious consequences. Or, if they are bigger mounts, they might trample allies, or fly away, removing your character from combat until you regain control.
Not sure if that's enough of a drawback to balance out the advantages of having the extra movement and extra attacks, but, it might be. Sure, you get all those cool goodies, but, someone tags you with an attack, you fail a Concentration check, and now your mount is out of control. Regaining control might be a Handle Animal check with a DC=the Concentration check DC.
A simple random table for losing control might look like this, with new checks made each round until the mount is controlled again:
1. Mount rears and bucks. Acrobatics check=Concentration Check DC to remain mounted.
2. Mount is Frightened. Will move away at best possible speed.
3. Mount randomly lashes out at nearby targets.
4. Mount bites rider.
5. Mount attempts to scrape rider off.
Or something like that. Won't come up all the time, but, could come up often enough to be significant.
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