Jeff Ibach has an unofficial Dungeons & Dragons supplement that fans will enjoy called Monstrosity Manual. This supplement includes 22 stat blocks for monsters from 3rd and 4th edition of D&D, all converted for 5th edition. The monsters range from CR 1-19 to provide a challenge for any party. Some of the included monsters are: Abyssal Eviscerator, Banelar Naga, Bright Naga, Deepspawn, Destrachan, Displacer Serpent, Fihyr, Great Fihyr, Herald of Rot, Krenshar, Lodestone Marauder, Mad Slasher, Swarm of Mad Slasher Young, Mad Slasher Alpha, Noble Tsochar, Tsochar Parasite, Phargion, Scarab Lamia, Swordwing, Thaskor, Voor, and the Walking Wall.
From D&D Editions past comes a collection of monstrous creatures for your 5th edition game!
You can purchase Monstrosity Manual from DMs Guild for $2.50.