Mammoth rider's pet

Is the mammoth rider restricted to the list or do those just increase the list that the character already has access to?
I'm asking because a person at my lodge is wanting to have a huge snake, getting the snake from the first mother fang cavalier archetype and then using mammoth rider to make it huge.
I'm aware of the rules for paladins, cavaliers, samurai, and rangers to be limited to their lists. And I think it's a general rule that you have to pick from the list and thus his snake doesn't work for mammoth rider, but I want to make sure and have supporting evidence before having to shut down his build idea.

Requirements

Special: Animal companion (or mount from a class feature that progresses as an animal companion) with at least 6 Hit Dice.
...
Steed (Ex): A mammoth rider can choose from among the following animal companions to serve as her loyal steed: amargasaurusB6, arsinoitheriumB2, aurochs, baluchitheriumB3, brontotheriumB6, camelB2, cat (big), deinotheriumB6, dire polar bearB5, elasmotheriumB6, elkB3, horse, kentrosaurusB6, mastodon, megalocerosB2, moaB5, rhinoceros, styracosaurusB4, triceratops, uintatheriumB5, wolf, woolly mammoth (use the statistics for a mastodon), or woolly rhinoceros. 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.

It looks like they meet the requirements for the Prestige Class, but the moment they take levels in it their snake would be replaced by one of the listed creatures.

Unless Tonya (the GM for the Campaign) expanded that list to include snakes as steeds. I don't see any rulings from the FAQ, Campaign Clarifications or Additional Resources. Maybe someone from Leadership made a ruling previously, but I am unaware of one.

Or at least that's how I read it. I'd personally allow it in a home campaign.

Just noticed that "bear" is not on that list, but I read recently that someone wanted to improve their Owlbear via Mammoth Rider, so that leads me to believe someone from Leadership did answer that question.

Maybe someone has a link?

The list of steed options got expanded in Adventurer's Guide:

Steed (Ex): A mammoth rider can choose from among the following animal companions to serve as her loyal steed: amargasaurus B6 , arsinoitherium B2 , aurochs, baluchitherium B3 , brontotherium B6 , camel B2 , cat (big), deinotherium B6 , dire polar bear B5 , elasmotherium B6 , elk B3 , horse, kentrosaurus B6 , mastodon, megaloceros B2 , moa B5 , rhinoceros, styracosaurus B4 , triceratops, uintatherium B5 , wolf, woolly mammoth (use the statistics for a mastodon), or woolly rhinoceros.

It should be noted though that the B5 dire polar bear is just a regular bear companion, not the nice new grizzly. It becomes Huge of course, but it's kinda feeble.

That's the same list I quoted, but indeed I didn't realize that "Dire Polar Bear" used "Bear" statistics.

You'd think they'd just list "Bear" to begin with -_-

That's the same list I quoted, but indeed I didn't realize that "Dire Polar Bear" used "Bear" statistics.

You'd think they'd just list "Bear" to begin with -_-

I don't know if it was my question that you're referring to Nefreet, but I was asking about a Huge Owlbear. I hadn't realized that the Dire Polar Bear used the base Bear Companion either.

My main question, "can His Majesty Snap Wyrmbane become Huge?" has been answered. It would be nice if he doesn't become noticeably weaker when it happens.

[...]

GRRRAAAARRRRRR!!!!*

* I'm the King in question, and I approve this message.

This thread is about a Mammoth Rider keeping their snake.