The classic Supermount build used a couple levels of Paladin, a level of Beastmaster, and fill the rest up with Halfling Outrider, Wild Plains Outrider, and Cavalier. Take the feat Devoted Tracker to count your Special Mount as your Animal Companion, take Natural Bond for a couple bonus effective Druid levels.
Most Supermounts use the feat from Draconomicon that lets you have a Dragon as a special mount, but that kind of kills the idea of riding a velociraptor.
Edit*
I should probably explain the results of this. Paladin gives you a Special Mount. Beastmaster gives you an animal companion at your druid level +3. Devoted Tracker lets these two be the same animal. Halfling Outrider advances both at the same time, letting you double dip for your mount's bonuses. WPO advances either your AC or your Special Mount abilities, but not both. Cavalier advances Special Mount only.
So you end up with this:
Effective Druid Level = 3 (Beastmaster) + 3 (Natural Bond) + 10 (Halfling Outrider) + 3 (Wild Plains Outrider) = 19
Effective Paladin Level = 5 (Paladin) + 10 (Halfling Outrider) + 1 (Cavalier) = 16
Then you look at the tables for Animal Companion bonuses and Special Mount Bonuses, and apply all of those to the same creature - in your case, a warbeast Fleshraker. Totals are 20 bonus Hit Dice (and all the BAB, feats, and skill points that entails), +22 natural armor, +10 Str, +6 Dex, a real Int score (so the bonus tricks are unnecessary), and a whole slew of special abilities.
Oh, and ask the DM what happens when an animal advances to Epic levels, because you now have over 20 hit dice.