Not actually all that common, but enough that at least some languages have a name specifically for them. Merlin tends to simply skip straight to describing himself as an incubus, though.

"So, are you enjoying this dream? Or would something else be more to your liking?"

... Not that there's really any point to him playing around tweaking a dreamscape to suit a machine, but, well, he's in a good mood with his powers back and feeling inclined to show off.