Jelly - That thing over there that looks like jelly? It's not a jelly - it's a blob! A blob is an extraterrestial predator that will eat you face for desert (after melting you gonads for the main course). Avoid jelly-like substances at all costs! Unless at a child's birthday party (The jelly probably will just be jelly in that case).
Jackal - Anyone that has a dog's head instead of a human one is an Egyptian Jackal. They will indeed try to kill you unless you can find that sweet spot on their bellies that makes their leg go crazy. Dog-people love that shit!
Janice - Character from friends that has the power to summon demons with her battle cry - which sounds to a typical human like a whiny, obnoxious laugh. Beware the Janice!
Jack in the box - Clowns are evil. Little tiny clowns that leap out of toy boxes are even more evil. Avoid!
Jackson Five - you know that family in Texas Chainsaw Massacre that like to kill and eat teenage flesh. This family is even more disfunctional. Any member of the Jackson Five family will fuck your shit up!
Jackhammer - If you have one during a horror film, great! If you are looking down the business end of one, prepare to have your face moved to the back of your head.
Jason Voorhees - Ex member the Boston Bruins ice hockey team from the 90s (who were known to e a little heavy handed). Due to constant head trauma from multiple body checks, Jason is only able to see hockey pucks where people's faces are. His natural instinct is the stab and poke at the puck in order to win the faceoff. This means death for anyone unlucky enough to be the puck.
Jamaica - They have a Jamaican bobsled team! There is nothing else they can provide you during a horror movie situation other than a Bob Marley backing track and a tea-cosy hat.
Jiminy Cricket - After the ordeal of helping Pinnochio, Jiminy went on to earn the black arts and is now a practising necromancer in Wisconsin.
Jorge Bush - The ex-US president's mexican cousin. Is the possible proprietor of the Dusk till Dawn nightclub just past the border.
Jennifer Aniston - Little do people know, but this ex Friend's actress is a know Leprechaun killer. If you find yourself up against a little Irish demon, then you should call Jennifer Aniston. She won't return your calls though, because she's famous.
J-Lo - The leader of the Martian nation of DeBloK; a species of humanoid creatures that house their enormous egos inside of a hidden pouch inside of their humungous asses.