How does the Huntmaster Cavalier work with Mammoth Rider?

[jabberwoky]

The Huntmaster Cavalier archetype gains animal companions equal to his effective cavalier level. To become a Mammoth Rider, he needs an animal companion of at least 6th level. By the time the Huntmaster can become a Mammoth Rider (level 9), the Huntmaster may already have more than one animal companion.

At level one of Mammoth Rider, the character gains "Gigantic Steed": 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.

My question: If the Huntmaster/Mammoth Rider has more than one animal companion, do all of the Huntmaster's animal companions gain the benefits of "Gigantic Steed", or only those animal companions at 6th level or higher, or only one of the animal companions?

[Dr. Calvin Murgunstrumm]

Quote:

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 lose one companion and replace it with your new steed. Gigantic steed only applies to that steed.