BABEL FISH

 

"The Babel fish is small, yellow and leech-like, and probably the oddest thing in the Universe."

―The Hitchhiker's Guide to the Galaxy

This small non-sapient being lives in the brain of other creatures and feeds on "brainwave energy received not from its own carrier, but from those around it". By unintentionally converting sound waves into brain waves and excreting those into the mind of its host, it actually works as a universal translator and enables its carrier to understand any spoken language, no matter how strange.

Source: The Hitchhiker's Guide to the Glalaxy

BETELGEUSIANS

Betelgeusians are an advanced  space faring humanoid species native to the Betelgeuse planetary system, and known to inhabit at least two of the system's planets: Betelgeuse V and VII. Betelgeusians have strange familial structures, including multiple mothers and some sort of relation known as semi-cousin.They are known to sometimes get extra limbs and even heads attached to their body.
Source: The Hitchhikers's Guide to The Galaxy

MAGRATHEANS

Planet of Origin: Magrathea, located in orbit around the twin suns Soulianis and Rahm in the heart of the Horsehead Nebula.

Magratheans are an advanced humanoid alien species which, during ancient times of the Galactic Empire, was known as the famed planet-builders; Hyperspatial engineers sucking matter through white holes in space to form dream planets as their advanced technology and artistic skills allowed them for other species according to the specifications of their clients. Ultimately, such a tremendously expensive and lucrative business accidentally resulted in the galactic economy being ruined and so the Magratheans went dormant for billions of years, waiting for it to recover. Meanwhile, their species, planet and history were slowly relegated to the status of mere legends.

Among the planets built by Magratheans was Earth; actually a supreme computer with an organic matrix requested by the Pan-Dimensionnal Mice, who used it to search for the ultimate question about life, the universe and everything else (the answer to which they already know to be 42). 

Source: The Hitchhiker's Guide to The Galaxy

VOGONS

Planet of Origin: Vogsphere, orbiting Vogsol
Vogons are three meters tall greenish bipeds who developed a highly bureaucratic civilisation in the distant pastr and then entered evolutive stagnation. Upon discovering the principles of interstellar travel, they migrated to the Megabrantis cluster, the political hub of the galaxy. They now form the powerful backbone of the Galactic Civil Service.
Source: Douglas Adams, The Hitchhiker's Guide to the Galaxy

STRANGULOUS STILETANS OF JAJAZIKSTACK / JAJAZIKSTCKANS

Planet of Origin: Jajazikstack (now destroyed)
The Strangulous Stilletans were an aggressive sentient species who lived some billions of years ago. They were wiped out by the Silastic Armourfiend of Striterax.
Source: Douglas Adams, The Hithchiker's Guide to the Galaxy

STRENUOUS GARFIGHTERS OF STUG

Planet of Origin: Stug (now destroyed)
The Strenuous Garfighters were an aggressive sentient species who lived some billions of years ago. They were wiped out by the Silastic Armourfiend of Striterax.
Source: Douglas Adams, The Hithchiker's Guide to the Galaxy

SILASTIC ARMOURFIENDS OF STRITERAX / STRITERAXIANS

Planet of Origin: Striterax (now destroyed)
The Silastic Armourfiends were a sentient species who lived some billions of years ago. They were so aggressive that without adversaries, they were could resort to automutilation until they become unconscious. They wrecked the surface of their planet, forcing the surviving population to live in bunkers deep below the surface.
During one of their wars, they asked the great computer Hactar to design them the ultimate weapon. Hactar created a hand-held bomb which would connect the core of every major sun via hyperspace, destroying the entire universe. They attempted to use the bomb to blow up a munitions dump, only to realise that Hactar had built a dud weapon since it could not conceive of an occasion when the use of such a weapon could be justified.
Eventually, after defeating the Strenuous Garfighters of Stug and the Strangulous Stilletans of Jajazikstack, they wiped themselves out.
Source: Douglas Adams, The Hithchiker's Guide to the Galaxy