PAIRANS

Planet of Origin: Paira, a planet on the same orbit as Earth but on the opposite side of the sun

The pairans are a pacific sentient non-humanoid species which developed an advanced civilisation capable of interplanetary space travel millenia ago and have been watching Earth for 4000 years. They look like 2 meters tall starfishes standing up on their 2 bottom legs and using their 2 middle legs for manipulation. A giant eye at the center of their body serves as their main sensory organ.

Pairan technology allows them to transmutate into another lifeform, a technique they used with humanity when they made contact with selected humans in 1956 to warn mankind of the danger of Planet R, a comet on collision course with our own as well as of the nulcear research, mainly the development of Urium 101, a substance much more powerful than regualr radioactive elements.The comet was finally destroyed thanks to the use of Urium 101 and the Pairans returned to their homeworld.

Source: Warning from Space or Mysterious Satellite (宇宙人東京に現わる Uchūjin Tokyo ni arawaru^?, Spacemen Appear in Tokyo, Diaie, 1956. Director: Koji Shima, Writers: Gentaro Nakajima (novel), Hideo Oguni (screenplay). Jay Cipes and Edward Palmer (english dialogues)

LEGION

Legion is a silicon based colonial lifeform or elephant size insects lead by gigantic queens (140 meters high /600 tons heavy).
The colonies expand from world to world by growing huge biological flowerlike launchpads which, upon explosion, catapult seeds into space that will colonize other worlds.
The queens are equiped with an EM beam that is fired from the nasal horn when it is opened ; The extra limbs or "crown" around legion's head can produce and Electro-magnetic shield that can deflect most kinetic attacks. Also, if the nasal horn is torn or blasted off, Legion can produce whip-like lasers that can impail and cut foes. Legion also has the ability to fly, despite it's enormous size. Legion can also summon and produce up to 100 Symbiotic legion from the egg cases on her abdomen.
Source: Gamera 2: Attack of Legion (ガメラ2 レギオン襲来, Gamera Tsū: Region Shūrai, Gamera 2: Legion Advent)

VIRAS

A space faring civilisation from a system not too far from Earth. Grey colored, Virans somewhat resemble six tentacled squids that are able to stand up right and walk on the ground. They have head with a pair of foreword-facing eyes, an owl-like beak and three additional tentacles with razor sharp points sprouting from their head. Amphibious by nature, they can breathe as well in Earth's atmosphere as underwater. As a special defense, If need be, Virasn have the uncommon ability, when a group gathers for that purpose, to fuse together into a larger version of themselves. The limitations of that fusion are not known but fused virasn have been known to be as tall as 50 meters high. They can also clasp together and stiffen their head tentacles into a solid, razor sharp point.
Virans have the abilities, acquired on innate, to merge together into a giant specimen.Viras civilisation made two unsuccessful attempts at invading Earth in 1968. It is their only known recorded contact with mankind.
Source: Gamera vs. Viras (ガメラ対宇宙怪獣バイラス, Gamera tai Uchu Kaijū Bairasu?, Gamera Versus Space Monster Viras)

ZIGRANS

Planet of origin: Zigra

Zigrans are an alien species who had already developed interstellar space travel in the late 1960ies. They are amphibious and may somehow look like a silvery grey armoured swordfish or a goblin shark, with a pointed nose, a row of sharp fins on his back, some pectoral fins and glowing red eyes . They can swim with their fins or stand upright on their hind fins/legs.

A zigran scout came to Earth in the late 20th century with the mission of preparing the planet for an all out invasion. Zigrans had the intention of enslaving the human race and raising them like cattle as he came from a planet where instead of people eating fish, fish eat people. When his saucer was destroyed by the kaiju Gamera, the scout, known as Zigra, came into contact with Earth polluted sea water which mutated him into giant size.

In kaiju form, Zigra's combat style is geared towards brutal melee combat. He is able to fire beams that can paralyse an opponent, and then follow through with vicious slashes with his sharp fins. Zigra can also walk somewhat awkwardly on his tail fins.

He was featured in Gamera vs. Zigra, the sixth Gamera film. Like the alien Viras did two films earlier, Zigra used a pair of human children as hostage in order to force the human race to do his biddings, but Gamera engaged him in battle. Gamera eventually threw him on land where the alien was weaker and used a rock to play his theme song on Zigra's dorsal fins like a xylophone before setting it ablaze with his fire breath.

Source: Gamera Universe, Gamera Tai Zigra