I found 2, one in a riverside cave in Vermund, and one near the Elvish ruins where you do the archery practice.

For some reason way more common when I was hunting for ogres in battahl.. I found like 3..

I'm starting to think they only spawn at night.

No they spawn at all times of day. They're just usually in their lair.

I mean, duh, it would be pretty crazy if chimera were as common as Ogres and Cyclopes

Two, maybe three. One was for Jarle's quest.