I remember that I thought they were robots rather early, there was some clue I can't remember now (I'll try tomorrow). Though, in all likelyhood, when I say "early" it might be the same place in text at which everybody guessed.

EDITED: yes. I found that quote. As I expected, it was when everyone else must have guessed, too - when Eddie talks to Gran-pere.

Eddie Dean's eyes widened. His first thought was that he now understood about the horses—all the gray horses. His second was Of course. It makes perfect sense. We should have known.

Robots were the only thing I could think of that fit that reaction, both logically and emotionally. There just was nothing more left among the main concepts of human, quasi-human and mythological beings that would have produced this kind of answer from Eddie (given all his previous experience of the quest, with its recurrent mad technology motif), and explain the horses at the same time.