Comments by Date (2)
These appear to have a pretty decent drop rate - I've gotten 2 in 4 runs. You can see and learn these even if you already know one.
They don't appear to spawn during the day. I believe I saw some starting at about 9 or 10 PM. However, they seemed to respawn fairly quickly, unlike the Shadeling. This is only one day's worth of data, so no hard numbers yet.
EDIT: Just saw a bunch of them spawned at 1 PM server, so they don't only exist at night.