Steed (Ex)
A mammoth rider can choose from among the following animal companions to serve as her loyal steed:
Arsinoitherium, aurochs, baluchitherium, camel, cat (big), elk, horse, mastodon (elephant), megaloceros, rhinoceros, triceratops, wolf, or woolly mammoth (use stats for a mastodon).
A GM might expand this list to include other possible steeds. This steed functions as a druid's animal companion, replacing any animal companion or mount gained from another class. The mammoth rider's class levels stack with other classes that grant an animal companion to determine her effective druid level.
You have to pick from that list it looks like.
Oh. My bad. Well, thanks.
The game isn't too big on fantasy mounts unfortunately, unless you go 3rd party or houserule things. That said, its not a bad idea to do so if it makes your game that much more fun.