IO

(If the name refers to an individual of the species whihc was captured on Earth or if it refers to the species itself remains unknown. We will treat it as such here)

Io are 3 meters tall crustacean humanoids who developed a space faring civilisation. Io are covered in thick carapace. They possess 2 digits on their feet and hands. Females are red but it remains unknown if males are of the same color.

An individual from this species was kept prisoner on Earth by the US Army during the later part of the twentiest century.

Source: Escape from Planet Earth.

BETELGEUSIAN CRABMEN

Planet of Origin: Supposedly in planet orbiting Betelgeuse

The Crabmen are a vaguely humanoid crustacean species that developed a spacefaring civilisation as of the 4th millenium. They possess large pincer-like hands and antennas that can spew a deadly green acid and had a geometric language.

Source: Beyond Infinity, by Nat Schachner Aka Chan Corbett, in Astounding Stories, Jan 1937.

The story tells of a series of invasion that befell Earth starting in 3195, as the universe unexpectedly starts to collapse on itself and the surviving species rush toward the worlds who are still unaffected by the phenomenon to delay their extinction. The Betelgeusians were the second of a wave of invaders  that attacked Earth, arriving in 3241, as a migration fleet of 100 000 spaceships fleeing their homeworld's destruction. They were defeated by mold and spores which attacked the softr skin under their shells. They nevertheless reduces the Earth population to 2 billions people.

LOBSTER MAN

Planet of Origin: A water planet in the Milky Way,
The aliens known known as Lobster Men are sentient crustacean humanoids which develop an advanced civilisation capable of space travel. While roughly anthropoid in shape and possessing a face with disturbingly human features surrounded by a beardlike hair collar, the Lobster men are covered by chitin plates, have loster-like pincers instead of hands which are strong enough to cut through steal at the end of long arms as well as small lobster-like legs coming of their arms, legs and forehead. If those smaller limbs are functional has not been witnessed. They also possess lobster-like antennas.

The lobster man have been watching Earth for "as long as men have talked out loud" and are known to have sent an emissary to Earth as a scout for an invasion at the end of the 20th century. The scout tried to cause a nuclar explosion to wreak havok, an effort that was thrwarted by the crew of an american submarine.

Source: Voyage to the Bottom of the Sea, The Lobster Man, directed by Jus Aldiss, written by Al Gail and Irwin Allen.

The Lobster man's spaceship.