Dark Hounds appear whatever time of day. There is a named one - Howler - that drops Remains that can be stuffed. Preumably he can be popped by killing the DH placeholders, but don't hold your breath.
On Gaheris I've levelled from 23 to 35 (so far) and haven't got him to spawn.
At l35 most DHs are green or blue con with an occasional yellow.