the only thing i can add (because i went looking too), is an example of an NPC in a 3e book with a mount. the only one i found that would really apply is Scyllua Darkhope (FRCS p164)
she's a blackguard and with her levels, her mount (a nightmare)should have been advanced 4 HD, +3 AC, etc. and so on.
in addition to these add ons, the nightmare has an extra feat (iron will) and a ton of extra skill points. if you look at creature advancement in the MM, an outsider should have one extra feat per 4 HD and 8+Int mod skill points per extra HD
so it looks to me like a mount does advance per the MM creature advancement rules, with the noted exception of d8 HD and not d10 (although +1 skill point per HD for a magical beast is pretty silly, i mean, why bother?)