Dragon

Not to be confused with the dragon-like antagonist, Ridley.

Dragons are a species found primarily on Zebes, specifically in magma filled areas. They are impervious to molten rock, and they have the ability to launch balls of fire from their mouths. Their main method of attack is to hide themselves in magma, and ambush prey when they are close. They resemble a reptilian seahorse.

Metroid
"These ancient Dragons live in the lava and stick their heads out to blow fire at Samus. Beware! If Samus is hit by their fire blasts, his energy weakens."

Dragons are found mainly in Norfair and Ridley's Lair. They hide in lava in order to surprise and attack Samus. They are red in appearance.

Super Metroid
"These enemies lives [sic] in lava, and spit out fireballs."

There are 2 types of Dragons, Green (found in Crateria) and Red (found in Norfair). They hide deep underground so they are not visible and they will attack Samus viciously with fireballs if they detect her.

A bipedal version of this creature is present in Norfair, and is called Alcoon. These can jump from solid ground to attack. The Wave Beam can damage it when it still resides within the rock, killing it before it even attacks.

Metroid: Zero Mission
Dragons also appear in Zero Mission as well. They reside in Norfair. Their method of attack is spitting blobs of lava at Samus. In Hard Mode they appear in areas where they didn't in other modes.

Trivia
The Dragons look similar to the Dragonzamasu[] from Super Mario Land.