Well I guess its more of an assumption on my part. Because its said to have guarded an ancient city. Plus there's Golurk.
Golurk is based off of the golem (clay statue brought to life), so I can understand, but it seems to be that Sigilyph is a Pokemon that's just been trained to the point where guarding their city became instinct and it passed onto their descendents.
"Murder of the living is tragic, but murder of the idea is unforgivable." - Janus, speaker of the synod