Carl Cramér replied:
"The 3.0 unicorn had this text "They either charge, impaling foes with their horns like lances (see Charge, page 124in the
player's handbook),
or ..." The rules reference is to charge, not to the lance, but to me
it implies that this uses the rules of a lance charge, not just a horn
charge that is similar in execution to a lance. NB that this is in a
rules block, not the flavor text. The Face/Reach section of the stat
block also mentions a 10 ft. reach with the horn alone, like a lance.
This is not flavor text.
Now, this is 3.0, not 3.5 or
Pathfinder, but the OP was asking for anything to base a judgement on,
and this is the one case I could come up with where a creature has
different kinds of natural reach. If we can come up with a Pathfinder
creature that has several kinds of natural reach, that might enlighten
us further. The only one I can find at short notice is the animated
object, and that only as a special option with no notes attached.
I
still think the OP is wrong and his GM right by RAW. The charge rule
says " You must move to the closest space from which you can attack the
opponent.", which is obviously a square at reach, because the eidolon
does have reach."
Sorry for simply copying this, but it didn't appear in the forum, only in my mailbox.
Well, i had no idea in the 3.0 edition unicorn, i'm gonna take your word for it.
But let's see another example: I have in my one hand a longsword and on the other a whip, a reach weapon, and a 15 ft. reach at that. What rule denies me to make a charge attempt with the longsword instead of the whip? In any other case, when you have ready more than one weapons, you can choose what weapon to use/make an attack with. That is far more true in the case of a monk making a flurry of blows.
Does that mean that i'll have to sheath or drop my whip, so i can get closer and hit my target with my longsword?