The tentacles are
sentient, triangle-shaped beings. They were first introduced in
LucasFilm Games' classic
adventure game,
Maniac Mansion, and appeared again (with even more
personality) in the
sequel,
Day of the Tentacle.
They are an experimental creation of the maniacal scientist Dr. Fred. Imagine cutting off the tentacle of an octopus, giving it an eye and a huge suction cup for feet, and have it walk around and talk.
Especially two tentacles are well-renowned to adventure game geeks around the globe: The evil genius, Purple Tentacle, and the punk-rocking but deeply sympathetic Green Tentacle.