The Displacer Beast is a long-running monster from the Dungeons & Dragons game. A rip-off of a monster from a 1939 short science fiction story (A.E. van Vogt's "Black Destroyer") called the Coeurl, this monster resembles an oversized panther with extra legs (two or three sets of forelimbs, one pair of hindlimbs), and squid-like tentacles sprouting from its shoulders. Its name stems from an innate affinity for illusion magic; displacer beasts can "displace" their image, producing an illusory duplicate of themselves that mirrors everything they do, but which is positioned several feet away from where it actually is. Thus, melee attackers are tricked into aiming at the wrong spot whilst coming within reach of its wickedly hooked tentacles.
The displacer beast has been in literally every single D&D edition that's ever existed; first appearing in the Greyhawk supplement in 1975, it appeared in both Advanced D&D (Monster Manual for 1e, Monstrous Compendium Volume 1 and Monstrous Manual for 2e) and in Basic/Expert (Basic Set, Expert Set, D&D Game Set, Rules Cyclopedia, Adventure Game Set). From 3rd edition through to 5th, it appeared in the first Monster Manual for each edition.
They had an "Ecology of the Displacer Beast" article for AD&D 1e in Dragon Magazine #109.
Displacer beasts are usually described as evil, savage monsters; sapient as any humanoid, but with a perverse love of killing and inflicting suffering. The one exception is in 4e, where they are, whilst still dangerous, less outright evil - the 4e version is actually a fey creature, a magical big cat originating from the Feywild. In other editions, it's a magical beast and just happens to be running around in your campaign setting.