Jekyllstein

Jekyllstein is the slash ship between Dr. Henry Jekyll and Victor Frankenstein from the Frankenstein, Jekyll and Hyde, Once Upon A Time and Penny Dreadful fandoms.

Canon
Jekyll and Frankenstein are both the titular characters of famous Gothic Horror novels. They're both scientists who push the boundaries of what would be considered acceptable in the field of science, Jekyll brews a potion that changes his appearance to let his darkest impulses loose without fear of consequence, Frankenstein tried to create a living being in an attempt to create the perfect human. In both cases, their experiments backfire as Jekyll's persona "Mr. Hyde" takes a life of its own, while Frankenstein's Creature turns against its creator. In both cases, their creation wreak havoc on society, Hyde just for his own amusement and the Creature for revenge against his "father". Lastly, both scientists refuse to take responsibility for their actions, resulting in their own deaths.

In both cases, they hide their experiments from their closest loved ones, fearing that they would be rejected by society if they came clean.

Once Upon A Time
Jekyll and Frankenstein stand out in a cast pulling from all kinds of different stories for being the only characters originating from horror novels. Despite both works being fairly similar, with the show's universe having characters from similar stories co-exist, Jekyll and Frankenstein are said to originate from different worlds. They both had amiable relationships with the main cast, but Frankenstein turned out to be genuinely benevolent while Jekyll had sinister intentions.

Penny Dreadful
Frankenstein is a main character in this series, the story has him get his long-time friend Dr. Jekyll involved. They are stated to study partners.

Fanon
Shipping the two mad scientists of Gothic Horror literature together has been thought of ever since the idea of crossing these stories had been introduced to the public. Many have noted that Jekyll makes an excellent foil to Frankenstein, as an accomplished scientist who is mostly in control as opposed to the struggling undergrad who frequently wallows in self-pity instead of taking responsibility for the Creature.

Once Upon A Time fans expected Jekyll to appear in a storyline surrounding the Land Without Color also featuring Frankenstein, feeling that there was a lot of potential surrounding said realm. Many still headcanon Jekyll as being born in the Land Without Color and that he and Frankenstein have a lot of unshown history, what helps is that Jekyll is only really shipped with Frankenstein (or himself) in that show's fandom.

Most of the popularity stems from the Penny Dreadful fandom. Though Frankenstein is shipped quite a lot in that series, Jekyllstein is the most popular ship for both characters. Much like Once Upon A Time, Jekyll is rarely shipped with anyone else, helped by him being more of a bit character for Frankenstein.

Fandom

 * FAN FICTION

Trivia

 * They both appear in the webcomic The Glass Scientists, with the twist that Frankenstein is a woman using the first name "Victoria" in this continuity.