Zygarde's capture rate is 3; the very same super-low catch rate every tough legendary ever has had, such as Mewtwo. Ya got lucky, is all.
... On the other hand, the cover legends are a joke to catch, as ya probably remember.
... Oh, but wander into some random grass on various routes and you'll eventually engage in a chase with somethin' rare. Assumin' ya haven't already.
... Well, Looker's themes are nice, at any rate.
The Looker sidequest had a decent mini-plot going on. It wasn't particularly good gameplay wise.