Leviathan

Leviathans were given this name in Metroid Prime 3: Corruption and is involved in the main story line.

A leviathan can be classified as a living creature, as it is born, grows up, and dies.

Purpose
The purpose of a leviathan is to spread phazon from the planet Phaazeto corrupt planets.

Actions
When a leviathan would land on a planet, it would still be alive for a matter of hours. Eventually, though, it would die off. Before it would die, it would find the biggest, natural predator of the area where the leviathan landed, and would corrupt the creature into protecting the remaining leviathan at all costs.

Pre Dark Samus
Before Dark Samuswas created, leviathans would, normally, grow in Phaaze untill they reached a point of maturity. Once this point has been reached, they would enter into a collection of leviathans. The older leviathans would be in the front lines, with the new leviathans being in the back.

Eventually, the old leviathans would launch off into space, hoping to find a planet to corrupt with phazon. Once the old leviathans left, the newer ones would move up, taking the old leviathan's place.

Dark Samus Controlled
Once Dark Samus returned(?) to Phaaze, somehow, she was able to control where and when a leviathan would launch and land.

All the actions of the leviathan are the same.

Where
Leviathans are created in a womb-like structure in the center of Phaaze.

Space Pirates
One leviathan hovers/ed over the Pirate Homeworld. It is/was the only way the Galactic Space Federation could enter into the dimension that Phaaze is in.

Metroid Prime
In the first prime, prior to Dark Samus, a meteor landed on Tallon IV and brought phazon to it.

It is believed that Metroid Prime is a Tallon Metroid corrupted to an extreme point by phazon.

It should be noted to support this that on the planet Phaaze itself, there are Phazon Metroids, Miniroids, and what appear to be shells resembling that of the one Metroid Prime had.