Here's the list I came up with of missing villains: Honest John and Gideon from Pinocchio, The Wolf from Peter and the Wolf, Lucifer from Cinderella, Tic Toc from Peter Pan, Horace and Jasper from Dalmatians, Edgar from Aristocats, The Sheriff of Nottingham from Robin Hood, Nero and Brutus the crocs from Rescuers, Amos Slade and/or Chief from Fox and the Hound? (Sort of villians, sort of just hunters doing their "job"), Sykes or his Doberman dogs from Oliver and Company, McLeach or even Joanna the lizard from Rescuers Down Under, Hyenas from Lion King, Clayton and Sabor from Tarzan, the Carnotaurus from Dinosaur, Rourke from Atlantis, Long John Silver from Treasure Planet, Alameda Slim from Home on the Range, Stabbington Brothers from Tangled, Turbo's true form from Wreck-It-Ralph, Bellwether or Lionheart from Zootopia, Te'Ka or Tamatoa from Moana, Queen Narissa from Enchanted, Lotso and Stinky Pete and Sid from Toy Story films, Hopper from Bug's Life, Morrdu the demon bear from Brave, Charles Muntz from UP, BUT I am super surprised they didn't do a Dragon Maleficent.
(As a joke, what about MAN from Bambi? A silhouette of a man, with fire as the background)