Methanogens belong to:

ARCHAEBACTERIA:

These archaebacteria are special since they live in some of the harshest habitats such as extreme salty areas (halophiles), hot springs (thermoacidophiles), and marshy areas (methanogens).
Archaebacteria differ from other bacteria in having a different cell wall structure and this feature is responsible for their survival in extreme conditions.

Archaebacteria are divided into three groups - methanogens, halophiles, and thermoacidophiles.

Methanogens: These bacteria are of marshy habitats. They are capable of converting CO₂, methanol, and formic acid into methane so named methanogens. These methanogens are present in the guts of several ruminant animals such as cows and buffaloes, and they are responsible for the production of methane (biogas) from the dung of these animals. These are chemoautotrophs.
Examples: Methanococcus, Methanobacterium, etc.