The villainess Erzsebet Ondrushko is based on Countess Erzsébet Báthory (Elizabeth Bathory in English) of Transylvania, who reportedly had three hundred young serving women put to death, in a human sacrifice cult in the early 1600s.
After the end credits, a short self-referential teaser for the next Hellboy Animated project is played. The working title for this, subsequently cancelled, third Hellboy Animated entry, was The Phantom Claw, and it was suppose to feature The Lobster, a popular spin-off character from the Hellboy comics. In the comics, The Lobster, a.k.a. Lobster Johnson, is a violent, deadly, and mysterious vigilante, who operated during the 1930s fighting gangsters, as well as supernatural threats. In the present day Hellboy timeline, the legend of The Lobster has gained popularity, and even cult status, thanks to the writings of a retired police detective, but the reading public doesn't know that his literary character is based on a real person.
Oliver Trombolt's character model bears a striking resemblance to David Xanatos from Gargoyles: The Goliath Chronicles (1994).
Hecate as an Iron Maiden form is one of the main recurring villains of Hellboy comics. The Iron Maiden is said to have been owned by Erzsébet Báthory, the historical analog of Erzsebet Ondrushko, even though Iron Maidens didn't exist in the 17th century.