The MM indicates that Sea Hags and Green Hags are both fey creatures while Night Hags are considered fiends.
Can anyone clarify if this is a typo and whether they're all supposed to be the same? If so, what should they be?
From page 178 of the MM:
“Night Hags were once creatures of the Feywild, but their foulness saw them exiled to Hades long ago, where they degenerated into fiends.”
So Night Hags are Fiends.
Volo's Guide to Monsters lists the Annis Hag and the Bheur Hag as fey, implying that hags are fey in general. The Night Hag appears to be an exception.