Handle Animal: Just a few ranks. Ride: max ranks.
Essential feats (assuming melee charge build): Mounted Combat, Trick Riding, Mounted Skirmisher, Boon Companion, Spirited Charge, Ride-by Attack.
Helpful feats: Power Attack, Wheeling Charge, Skill Focus (Ride), Lookout (along with your mount; it's a good teamwork feat). If your mount can obtain Dragon Style, that's handy. Rhino Charge is cool, but has some feat pre-reqs and most likely you AND your mount would need to take it to use it.
I would do 1, 2, or 6 levels of Sohei, perhaps 8 at most for the extra flurry attack. Definitely more ranger, though. At early levels, start off with ranger and dip Sohei for the really high level mounted feats especially Mounted Skirmisher. Probably Ranger 2 (get Trick Riding), then Sohei 1 or 2 (get Mounted Skirmisher, maybe something else), then back to ranger for a while. Just what I think would be best. Did you want to do Charging flurry w/ mounted skirmisher? If so, I guess after ranger 2 go sohei 6. If you had no plans to use flurry anyway, stick to 1-2 monk levels and consider also taking the Master of Many Styles archetype, as flurry won't be terribly useful for you.