I'd say let's allow them to treat armor spikes and/or spiked gauntlets as monk weapons.
OK, we seem to be getting somewhere with the Monk model.
I'd add gauntlets to the list, and give them kama as a weapon proficiency as well (they're practically identical to sickles, after all).
What about the Flurry of Blows idea?
If we say they can Flurry as a 4th or 5th level monk the Base Attack Bonus will match (3HD magical beast = +3 BAB = 4th or 5th level Monk), and it gets two attacks with gauntlets/kamas/spikes at -2 (4th level) or -1 (5th level) attack, each doing 1d6+3 (gauntlets) or 1d8+3 (kama/spike).
I'd go for 4th level, myself.
I would think that they would just get the Flurry of Blows attacks without any other monk abilities like improved Unarmed Strike damage, unless they actually get levels in Monk of course.
Should levels in Flurry stack? So a Houyhnhnm 2nd level Monk flurries as a 6th (+3/+3) or 7th (+4/+4) level one?
I'm also wondering whether it is wandering into Challenge Rating 3 territory. If we give it armour proficiency (light) and armour spiked studded leather, its full-attack damage is only a little weaker than an ogre and it has better saves plus much better speed and missile range.
Armor Class: 16 (-1 size, +1 Dex, +3 armour, +3 natural), touch 10, flat-footed 15
Full Attack: 2 hooves +5 melee (1d4+3) or armour spikes +3/+3 melee (1d8+3)[
maybe +4/+4 melee?] or light crossbow +3 melee (2d6)