If a Hippogriff Rider takes the prestige class Mammoth Rider, can the Ranger still use the Hippogriff as her mount? Does the Hippogriff grow huge and gain other benefits like the steeds listed in Mammoth Rider?
"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.
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."
"A GM might expand this list to include other possible steeds."
I.E. "If your GM says so, sure."