Not quite a "what breed", but more of a "why"

[Angeleyes707]

To go along with your horse analogy (I own horses as well) think of the Ameraucana as an APHA registered Paint, ignoring that most horses can be registered based on lineage in spite of breed standards, and the Easter Egger is somewhat of a grade pinto. Usually you can tell easily that a pinto doesn't have all the right genetics to be a paint based on body shape, facial skeletal ratios, etc. but sometimes it comes down to just not having quite enough white marking, or proof of lineage. It's a little different with chickens in that it tends to be more strictly based on features and not on lineage, but you get the gist.

Understandable analogy!