Degenerated Dragon

Degenerated Dragons are dragons that have since gone insane and lost all reasonining capabilities.

This fate was first predicted by the Dragon Elders after Dragon fertility mysteriously died out with the only known cure being to seal their true form in a stone, and assume a humanoid form to conserve power.

However many dragons were skeptical of this warning, and pridefully refused to give up their form, as such these dragons eventually degenerated into madness and ferality. These insane dragons were extremely violent without reason or cause, and were a danger to the entire continent.

The most dangerous of the Degenerated Dragons, the Earth Dragons, were battled by Naga and the Divine Dragons at great cost. Eventually they were sealed in the Dragon's table, with Fire Emblem securing the seal. In the remake, Shin Monshō no Nazo ~Hikari to Kage no Eiyū~, a great deal of other feral Dragons are also shown to be sealed with them.

Ingame
In the original Mystery of the Emblem, Degenerated dragons act similarily to transformed Manaketes, but did not start the battles as manaketes obviously.

In the remake, Shin Monshō no Nazo ~Hikari to Kage no Eiyū~., Degenerated dragons are a entirely different class then Manaketes. Degenerated Dragons use their species breath weapon to attack unlike Manaketes who rely on their dragonstones. In the harder difficulties, Degenerated Dragons use 1-2 range breath as opposed to 1 range.

Degenerated Dragons usually have higher caps then their Manakete counterparts as well in Shin Shin Monshō no Nazo. A Firestone using Manakete with Max Stats has 28 Strength, 24 skill, 24 Speed, 29 defense, and 24 resistance. While a Degenerated Fire Dragon has a 30 cap in all of those stats.