I'm pretty sure there was a method somewhere for helping a mount scale in level in spite of their rider's multiclass, but I can't seem to remember what it is or where I might find a method like that. Hoping that someone can help me find something.
For simplicity's sake, let's say the mount is a horse owned by a Cavalier/Fighter, (no using the Roughrider archetype). Anybody have any methods of helping the mount scale, whether partially or fully, with those fighter levels?
There is a feat called Boon Companion (from Animal Archive, I think) which adds +4 to your effective appropriate-class level up to a maximum of your character level. This can help to make up the difference to an extent, depending on how far you dip the multiclass.
Boom.
Aha! That's the one I was thinking of, excellent!
Boon Companion is pretty neat too. Could be useful at lower levels, or if I have less than 4 cavalier levels.
There is also the beast rider feat (yeah, same name as that one cavalier archetype...it is in fact confusing) for half and full orcs. That gives a +2 and can open up some more options (dinosaurs and large African herbivores). That stacks with boon companion it seems
And if you have a lot of feats to spare, you could try to get an animal companion outside of class abilities. By taking animal ally (needs nature soul), you basically get the ranger version of animal companions that uses your character level as a whole instead. Of course, you also have to drop yet another feat on boon companion, but eh. This could be useful, even for classes with access to ACs normally since you could just grab an archetype that trades it away.
I'd suggest taking Horse Master over Boon Companion regardless for any Cavalier. It is literally just a better version of Boon Companion (bar that it's Cavvy only), and leaves you free to multiclass later without fear of making Boon Companion a less desirable Feat.