Mordenkainen's Fiendish Folio
Mordenkainen's Fiendish Folio is a quasi-official PDF-only splatbook for Dungeons & Dragons 5th Edition, released on the DM's Guild. It is one of several such booklets produced by Wizards of the Coast to raise money for the charity "Extra Life". Currently, there is only one Fiendish Folio, but its titling scheme suggests a plane to release spin-offs, turning it into a series. As its name suggests, this is an update of assorted monsters from the original Fiend Folio, giving them new stats and lore.
Volume 1: Monsters Malevolent and Benign
Most of them are on the malevolent side of things.
This volume contains the following monsters: