Art by Ben WoottenThese appear in the upcoming Mordenkeinen's Tome of Foes according to this Dragon +video. I am not sure that Mystara or BECMI will be referenced in the 5E book, but this was one of the Nightshade monsters that originally appeared in the BECMI Master Set, by Frank Mentzer. These are some of the most terrifying creatures that existed in that edition. There were three types of Nightshade. Nightwing and Nightcrawlers were the two others. The Nightwing appeared the Immortals Triad novels by Douglas Niles.
Sea Elves is another Monster with long ties to Mystara and Classic D&D that also is said to appear in Mordenkeinen's Tome of Foes as are Nagpa.
Thanks to zontoxira at the Piazza for pointing me to the video.