I'd handle that with a wand of mount, actually. If you want a better combat companion, a plain old construct ought to be able to handle tireless transportation duties. I think "living" constructs will be limited to things that need to be smart.I wonder if the rules in the Eberron Campaign Settign will be extensive enough to create other types of living constructs.
A tireless horse is a good investment.
In any case, there are some hints at non-humanoid constructs. One leaked image shows an archer fighting what looks like a big mechanical lobster:

(ThoughI ask you, what self-respecting D&D fan would want to play in a setting with mechanical lobsters? That's not my D&D!)