New Monster High dolls are released regularly, but some characters in the Original Ghouls collection include Frankie Stein, Clawdeen Wolf and Draculaura. Cleo De Nile, Ghoulia Yelps and Lagoona Blue are also original characters. The dolls are inspired by classic monster characters and motifs, such as mummies, werewolves and vampires.
Mattel expanded the Monster High brand beyond the core characters to release other popular dolls, such as Abbey Bominable, Amanita Nightshade and Jinafire Long. The company also created characters from the same family or with related powers, most notably Clawdeen Wolf's pack members, Clawdia Wolf, Clawd Wolf and Howleen Wolf. Operetta is a character inspired by the Phantom of the Opera, but some of the doll's physical features, such as neck bolts, resemble Frankenstein's monster, similar to Frankie Stein.
Other dolls based on secondary characters include Rebecca Steam, Venus McFlytrap, Kiyomi Haunterly, Spectra Vondergeist and Twyla. The brand also includes several male dolls, such as Porter Geiss, Invisi Billy and Neighthan Rot. Beyond basic monster themes, many characters are named after the animals they resemble or other mythical concepts. For example, the Vandala Doubloons doll is styled as an undead pirate. Toralei Stripe and Catrine Demew have feline-inspired names, while Bonita Femur and Skelita are inspired by the Day of the Dead in Mexican culture.