I'm currently developing a druid (already being played) that I've got a pony with, that I intend to become more of an animal companion (much like the rangers 3rd level option) - I plan to awaken the horse (still quite a while away as I'm only level 6). However, I'm putting a headband of intellect on, so that should be interesting... :D
In terms of this kind of development, what are some good suggestions about RPing this particular situation, and any good home brew rules etc that could be used to go with it?
DM isn't too sure how to go with training it, nor turning it into a companion, as there isn't a clear way to do this without dipping into Ranger (which I'm trying to avoid at all costs)
I wouldn't worry too much about dipping into ranger for your pet. The way you should think about it is your pet is an NPC with its own free will. You could spend downtime training the creature to listen to certain commands. See what your dm thinks about running the pet as an NPC.