Animal ally & mammoth rider prestige class

[Humphry B ManWitch]

Can the animal ally feat which grants you an animal companion be the prerequisite for mammoth rider?

Can the mammoth rider choose something that is not in their list of steeds as a steed? As text is stating that it adds to the companion selection?

Assuming you can have a different animal say a snake viper for example will that now become huge?

[The Morphling]

No. It requires an animal companion class feature, not an animal companion. (Note: There is no such class feature called "animal companion" - just class features that grant animal companions.)

No. The text states that this replaces her old companion. The list of steeds is the only list she can use.

No. Your viper leaves, and you get a Rhino.

[Humphry B ManWitch]

Requirements
To qualify to become a mammoth rider, a character must fulfill all the following criteria.
Base Attack Bonus: +6.
Skills: Handle Animal 9 ranks, Ride 9 ranks, Survival 5 ranks.
Special: Animal companion class feature with an effective druid level of 6th or mount class feature that progresses as an animal companion with an effective druid level of 6th.

SKR has stated before that a feat can replicate a "class feature"

Steed (Ex): A mammoth rider can choose from among the following animal companions to serve as her loyal steed: arsinoitheriumB2, aurochs, baluchitheriumB3, camel, cat (big), elkB3, horse, mastodon, megalocerosB2, 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.

Can't see must here but I can see the word "Can"

Gigantic Steed (Ex): A mammoth rider’s steed increases to Huge size. The creature receives a –1 penalty on attack rolls and to AC, and a –2 penalty to Dexterity (to a minimum of 1); its base damage increases by one size
category; and its reach increases to 10 feet. It also gains a +2 size bonus to its Strength and Constitution. At 3rd level and every two levels thereafter, the Strength bonus increases by an additional +2; the bonus to Constitution increases by an additional +2 at 5th and again at 9th level.

Is the steed now a third type of animal companion.? Or is it just a new word for the same thing.

[cartmanbeck]

I actually disagree with Morphling... the animal companion class feature is the class feature of a druid, which is granted by a whole bunch of different archetypes, bloodlines, mysteries and feats at this point. It's referring to the class feature because that's how it's always referred to... based on your "effective druid level". So, I would say that Animal Ally does grant you the druid's animal companion class feature, and so would qualify you for the prestige class.

I do agree that you need to pick from the new list of animals given, as opposed to keeping something like a snake instead. The list of possible animals for the Steed class feature replaces the normal list of creatures that you can have as an animal companion.

[Samasboy1]

I definitely think the feat should work for pre-reqs.

I think you have to take an animal from the list. Steed replaces animal companion or mount. It says "You can choose from this list." So you are allowed no steed, or one from the list, but since you don't have an animal companion anymore you can't use that list.