So I'm sure in one of the endless Pathfinder books I read about either and Archetype or Order for the Cavalier, (maybe the Summoner) there was a summoner that used it's eidolon as it's mount. It started out saying that cavaliers did not consider them real cavaliers, and views them as a joke. (This of course is not word for word as i can't find it. ) I am 99.9% sure it was not 3rd party. I am almost completely sure it was in a book not on Paizo. Does anyone know what I'm talking about, and where to find it? Thanks
I found it! That was hard. And yep I was wrong 3pp. But at least Ive got it and I'm not going senile. Had to google "pathfinder cavalier order eidolons" in that exact order to find it:
Order of the Chimera
This order is exclusive, eclectic, and extremely odd, as it is composed solely of summoners who have chosen to adopt the trappings and mannerisms of cavaliers and ride their eidolons into battle as armored knights. […] It is generally treated with disdain by cavaliers of other orders, who see the Order of the Chimera as farcical and insulting, and not a true order[…]
This Reddit thread agrees that the only way to make a cavalier/summoner is multiclassing which causes issues with losing Evolution and spellcasting levels.
This other Reddit thread has some decent suggestions for making a Summoner that rides his Eidolon, which you could RP as a cavalier, just without the Challenge and inherent Ride bonuses.