As far as I can tell, it's not in the books
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:
Perhaps it is a Berbalang
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: