Imps  are a type of creature infamous for having a dangerous sense of humor and playing jokes on human beings.
Imps are lesser demons and seem to be paranatural, from a parallel dimension. There also appears to be more than one type of Imp. One book describes Imps as: "Invisible spirits with a dangerous sense of humor. The longer they remain free, the more powerful they become." 
The Ghostbusters encountered Imps on three separate occasions. On the first, Slimer accidentally disturbed one of Egon Spengler's dimensional conduit experiments. Six Imps were allowed to cross over to the physical plane. They proceeded to play pranks throughout the Firehouse until Ray Stantz identified them. Most of the Imps were also armed with pitchforks. Upon contact, the pitchfork emits an electronic sting.
Peter Venkman ran out of patience and decided to blast them with the Proton Pack. Instead, exposure to the Proton Stream caused one Imp to multiply into six. Overwhelmed by Imps, Egon was called back to the Firehouse from Canarsie, where he was visiting with Janine Melnitz' family. Peter, meanwhile, deduced that reversing the polarity on the Neutrona Wand would solve the problem. Instead, it forced every imp into a giant mass, manifesting as a Giant Imp Monster. The Ghostbusters headed to a power plant to boost a Proton Pack and trap it.
The second encounter was an Imp which worked with Morganan, a demon. He was also shown to be a nimble and laughing creature that only likes causing trouble as he told Morganan's fellow demons of his recent client. The Imp was kept in a chest until Morganan's Boss released it, much to Morganan's disdain. After it told other demons of Morganan's deal, Morganan imprisoned the demon in a glass case in retaliation.
On the third encounter, Ray had inherited his Uncle Gaylord's joke prop shop in Queens. Ray explored it and found a secret underground Hall of Humor, dedicated to the history of gags. Within this Hall lay a Little Black Box, origins unknown. The box imprisoned a pair of Imps. Ray opened it, unknowingly releasing them. For this type, they became more powerful the longer they remained in the outside world. The pair focused on the Ghostbusters and sent them on prank calls. The first was to the Hasty Pastry Bakery, then the Twin Towers project downtown, and at the waterfront near Pier 13. In each case, the Imps mischievousness grew more and more malevolent. Ray realized the black box was the only way to stop them and lured the pair back to Gaylord's Joke Shop. Once he bound them to the box, Ray trapped them.
The interdimensional Imps were tiny yet nimble entities, could jump to impressive heights, and when hit by a Proton Stream, multiplied sixfold.
The pair of Imps from Gaylord's Joke Shop could fly and turn invisible. They could also tap into phone lines and make a call. As they became stronger, their strength increased and could hurl fireballs of P.K. Energy. The Ecto Visor paragoggles allowed the Ghostbusters to establish a visual on them.  According to the P.K.E. Meter, in their fourth form, the Imp pair displayed a high level of spectral activity. Again, Proton Streams were ineffective.  For whatever reason, only the black box could neutralize them long enough for a trap to contain them.  Unlike other imps, these two are capable of speech.
Ray classifies Imps as Class 5 Focused Individual Miniature Proto-Plasmic Manifestations.  Focused means the Imps are tied to a specific locale where it appears and cannot stray from it. It appears a boost in energy allows them to roam. Proto-Plasmic Manifestation is a synonym for Corporeal Entities.
Based on LegendEdit
Imps come from Germanic folklore and are considered small lesser demons that with a wild and uncontrollable personality. They are fond of pranks and misleading people but ultimately seek their friendship. Imps were thought to be immortal but could be harmed by magic. They also served warlocks and witches, often bound to some object.
In "The Joke's on Ray," Peter asks what an Imp is;  despite having already encountered them in "Janine's Day Off" and "Chicken, He Clucked" of his question were to be interpreted literally.
- On page 12 of Ghostbusters Issue #1, an Imp from "Janine's Day Off" makes a non-canon cameo next to Janine's computer.
- On the Second Printing cover of Ghostbusters Issue #2, several Imps from "Janine's Day Off" appear.
- On page seven of Ghostbusters Issue #4, an Imp from "Janine's Day Off" appears next to Janine's flowers.
- On Cover B of Ghostbusters Issue #10, Imps make a non-canon cameo on Janine's desk and computer.
- On seven one of Ghostbusters Issue #13, Imps from "Janine's Day Off" makes a non-canon cameo between Ray and Winston and Winston and Roger on one panel and on Janine's computer in the last panel.
- On page five of Ghostbusters Issue #14, an Imp makes a non-canon cameo on the coffee pot in the kitchen.
- On cover A of Ghostbusters Volume 2 Issue #6, Imps makes a non-canon cameo.
- On page 14 of Ghostbusters Volume 2 Issue #15, an Imp makes a non-canon cameo in Peter's Wise Natural Potato Chips bag.
- On page seven of Ghostbusters Volume 2 Issue #18, the Imp from "Chicken, He Clucked" makes a non-canon cameo under a table.
- On page 20 of Volume 2 Issue #18, an Imp from "Janine's Day Off" makes a non-canon cameo under the computer.
- On page 16 of Teenage Mutant Ninja Turtles/Ghostbusters Issue #4, an Imp makes a non-canon cameo in panel 2, above April O'Neil's word balloon.
- On the Convention Cover of Ghostbusters: Get Real Issue #1, Imps make a non-canon cameo in the bottom corners.
- On page 14 panel 4 of Ghostbusters: Get Real Issue #3, the Imp with blue eyes, snout, and short horns makes a non-canon cameo to the left of the Miniature Inverter.
- On page 15 panel 1, of Ghostbusters International #4, the Unknown Soldier Ghosts' new forms appear to be loosely based on the later forms of the Imps from The Real Ghotbusters episode "The Joke's on Ray"
- On page 2 of Ghostbusters International #8, in panel 5, in the hole in wall left of Kylie is an Imp "Janine's Day Off"
- On page 8 of Ghostbusters 101 #2, in panel 3, next to the frame is an Imp from "Janine's Day Off". In panel 4, on Janine's computer screen, is the "Hi Toots..." repeating message made by the Imps in the same episode.
- ↑ Ray Stantz (2009). The Real Ghostbusters - "Janine's Day Off" (1987) (DVD ts. 5:13-5:14). Time Life Entertainment. Ray says: "An Imp!"
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Joke's on Ray" (1988) (DVD ts. 14:44-14:52). Time Life Entertainment. Ray says: "Imps are invisible creatures with a dangerous sense of humor. The longer they remain free, the more powerful they become."
- ↑ Egon Spengler (2009). The Real Ghostbusters - "The Joke's on Ray" (1988) (DVD ts. 14:57-15:00). Time Life Entertainment. Egon says: "With these Ecto-Visors, we can see just fine."
- ↑ Egon Spengler (2009). The Real Ghostbusters - "The Joke's on Ray" (1988) (DVD ts. 18:26-18:29). Time Life Entertainment. Egon says: "Peter, our throwers are useless against them."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "The Joke's on Ray" (1988) (DVD ts. 18:32-18:36). Time Life Entertainment. Ray says: "Wait, the Little Black Box... it's the only way we can trap them."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "Janine's Day Off" (1987) (DVD ts. 5:07-5:12). Time Life Entertainment. Ray says: "There's a Class 5 Focused Individual Miniature Protoplasmic Manifestation on the kitchen counter!"
- ↑ Peter Venkman (2009). The Real Ghostbusters - "The Joke's on Ray" (1988) (DVD ts. 14:33-14:34). Time Life Entertainment. Peter says: "What are Imps?"