Lenticular clouds also called UFO clouds because well, they look like UFOs. It's quite an uncommon phenomenon.
Most of the time they form above mountainous areas when moist air condenses but they can also form in other places with the presence of shear winds.
Here is a gallery of lenticular clouds submitted by various people on Earth Sky and a little bit more information on how these clouds form.
(Images credit: Yaping Wu/Wikimedia Commons; Omnisource5/Wikimedia Commons)