How to maximize survival of a mount?

[amatulic]

I'm building a character (not a Paladin) who has Ride as a class skill.

This isn't a combat character, but more of a party-support character. She's a small race, so her mobility on foot is slow. She needs a higher speed for her function in the party, and can't afford magical boots.

So, I'd like to give her a mount such as a riding dog, donkey, or war pony - all of which would give her a higher speed and let her keep up with the party.

A mount works fine for characters up to about 3rd level. AC doesn't really grow much past AC 20 or so, and AC begins losing effectiveness after about 5th level, where you start taking more hits. After that point it's more important to have high hit points than high AC, so you can absorb more damage, because you will get hit more often.

So my questions are:

• Does my mount, who shares the same experiences with me that gain me XP, also gain HP as I gain levels? Or is a war pony stuck at 13 HP his whole (short) life?
• If not, does it make sense for a race like a human or gnome to be able to absorb far more damage at higher levels than his horse??
• Is there any way to avoid the mount becoming essentially a single-use item that's unlikely to last through 1 encounter at 6th-7th level? It's hard to rely on something that won't last. And I'd rather avoid bringing a whole herd of spare mounts into the dungeon, although the other party members may find this useful for riding and carrying loot.
• Are there any rules covering leveling-up of mounts for non-Paladins?

Thanks. In the years I've been playing, nobody in my gaming group has ever brought a mount into the game. This would be something new, if I can make it work. But it seems that mounts are only effective at low levels of play.
-A