Rare monsters seem to spawn randomly or specifically in various locations. Though, the golem was controlled by magic (generally golem's are), but I would think that if it wasn't a random spawn spawn then it wasn't simply protecting an object on his own and probably had a controller, probably a boss, sent the golem once they noticed an explorer at the entrance (or created the golem for that purpose, even if the creator doesn't exist anymore). I really couldn't see a monster that strong and rare being sent to the entrance unless the boss at the end of the cave was a [much] higher level, since it'd be rather odd for the strongest monster to be sent to face those who just entered. The golem was only about level 15 (give or take), and it's not so unbelievable that Akihiko would make a cave, and a somewhat noticeable one at that, a death trap.
So I guess I'm with you, then. I was actually reluctant to say anything because I had considered it, and from a logical point of view it being a guardian made sense, but...I guess I was partially undecided on the idea and needed a little push. And sorry about these long messages, this'll hopefully be the last one -.-