I was looking at building a mounted combat focused summoner who rides in battle on his eidolon. I was looking at the Ride-by Attack feat and wanted to know if my eidolon could attack too. I found an old thread that discussed this with a normal mount. In that thread it was stated the mount could not Spring Attack on a Charge because they were both full actions. At first glance I thought this would be the same for a Summoner with a Trample equipped eidolon, but now I'm not so sure. (All the bolds are belong to me.)
An eidolon gains the ability to crush its foes underfoot, gaining the trample ability. As a full-round action, the eidolon can overrun any creature that is at least one size smaller than itself. This works like the overrun combat maneuver, but the eidolon does not need to make a check, it merely has to move over opponents in its path. The creatures take 1d6 points of damage (1d8 if Large, 2d6 if Huge), plus 1-1/2 times the eidolon’s Strength modifier. Targets of the trample can make attacks of opportunity at a –4 penalty. If a target forgoes the attack of opportunity, it can make a Reflex save for half damage. The DC of this save is 10 + 1/2 the eidolon’s HD + the eidolon’s Strength modifier. A trampling eidolon can only deal trampling damage to a creature once per round. This evolution is only available to eidolons of the biped or quadruped base forms.
As a standard action, taken during your move or as part of a charge, you can attempt to overrun your target, moving through its square. You can only overrun an opponent who is no more than one size category larger than you. If you do not have the Improved Overrun feat, or a similar ability, initiating an overrun provokes an attack of opportunity from the target of your maneuver. If your overrun attempt fails, you stop in the space directly in front of the opponent, or the nearest open space in front of the creature if there are other creatures occupying that space.
My reading of this is the eidolon could use his trample evolution and I could use Ride-by Attack while we go charging by (over) the target. Now granted this would be limited to trampling targets one size or more smaller than the eidolon, but otherwise is this correct?
You are correct.
You charge and as part of the charge you can trample everything on your way. Ride by attack modifies your charge and let you move after the charge. No problem until now.
The target of the charge can not trampled because the trample action ends on the charge target. The extra movement after the charge via ride by attack is without trampling.
Thats my interpretation of this ability combination.