On the back of the Mordenkainen's Tome of Foes book, there is a small creature with wings:
What is this creature? It doesn't match any of the creatures in any of the monster manuals. It kind of looks like an imp, but it clearly isn't one, and my only other guess is that maybe it's some kind of homunculus?
Looking through the pictures in Mordenkainen's Tome of Foes it most looks like the Berbalang.
Wizards of the coast has been known to put baby pictures of monsters in their books.
As such I suspect it's a baby Berbalang. For reference a Berbalang looks like this:
Most likely a Homunculus.
Since it has wings, it's not a quasit. As it doesn't have a tail with a stinger, it's not an imp. It could possibly be some kind of mephit, but it doesn't have any clear elemental appearance.
This leaves homunculus as the most likely tiny gargoyle-looking creature.
After scanning through Volo’s Guide to Monsters, The Monster Manual, Mordenkainen’s Tome of Foes, The Sword Coast Adventurer’s Guide, Guild Masters’ Guide to Ravnica and Wayfinder’s Guide to Ebberon, I can safely say that specific creature is not in any of those books.
However, there is one other place that creature is found - it's on the back of the Mordenkainen’s Tome of Foes variant:
Comparing the image on the variant back cover of Mordenkainen’s Tome of Foes to the image provided by illustro, I believe that this is a Berbalang:
If the creature on the back of the variant cover is indeed a Berbalang, we may assume that the one on the standard cover is also a Berbalang, though in a different art style. Similar things have been done before, compare the beholder on the standard cover to Xanathar’s Guide to Everything to the one on the variant cover:
It's acting like a gremlin. Berbalangs don't seem to have any interest in breaking things. Gremlin batspawn is the right color, shape, proportion and would be likely to be doing what that little guy is doing.
