Eidolon Mount

Your eidolon is capable of serving you as a combat-trained mount. If the eidolon's base form is not quadruped or serpentine, it also gains the ability to transmute its physical body into a form suitable for you to ride. This functions as the change shape ability, except the eidolon's base form changes to either quadruped or serpentine and its size changes to be one size category larger than its summoner's base size. Unlike other changes to size, this ability doesn't increase the eidolon's ability scores, reach, or weapon damage beyond that of its true form. (However, if the eidolon's size is reduced by this ability, its reach and weapon damage are adjusted accordingly.) If the eidolon's base form is not quadruped or serpentine when it gains this feat, it chooses which base form (quadruped or serpentine) to assume when using this ability. The eidolon cannot choose a base form that is not available to its subtype with this ability. If both base forms are available to the eidolon's subtype, it can change which base form it assumes when using this ability whenever it gains a new Hit Die. The eidolon can select evolutions that have either base form as a requirement, but any evolutions that require one of the base forms (but not the other) provide no benefit while the eidolon is assuming the shape of its other base form.

Prerequisites

Eidolon able to choose quadruped or serpentine as a base form