If you want a worm go get sandstorm and get an ashworm. It's like a mini-pruple worm, but with more awesome. Comes with it's own prestige class, the only 10 level mounted PrC I've seen that doesn't just focus on one thing. You essentially get the standard mounted feats for free, or upgrades to them if you already did, along with the ability to make infinite ride checks to negate damage, go with your mount while burrowing, and pump the DC of the poison attack.