Trip (Ex)
Source: Advanced Player's Guide, p. 63
An eidolon becomes adept at knocking foes to the ground with its bite, granting it a trip attack. Whenever the eidolon makes a successful bite attack of the selected type, it can attempt a free combat maneuver check. If successful, the target is knocked prone. If the check fails, the eidolon is not tripped in return. This ability only works on creatures of a size equal to or smaller than the eidolon. The eidolon must possess the bite evolution to select this evolution.