Fire Terrors are similar to normal Night Terrors, but they are red in color. Their eyes also glow with red bioluminescence.
Fire Terrors live in the caves inside Caldera Cay, and likely eat the fire it produces. Like the regular Night Terrors, Fire Terrors live and hunt in packs, and they have a fierce instinct to protect the Eruptodon egg, even from humans. They also share similar mannerisms as regular Night Terrors. Fire Terrors are also known to consume fire, as one ate the fire produced by Inferno. They also can use this fire against their attacker by using it as a shot.
These dragons are likely trained in a similar manner to a normal Night Terror. For the ones on Caldera Cay, it is possible they were once trained many years ago by Mala's family. Each generation of Fire Terrors passed on the trait of protecting the egg of the Eruptodon, as the drawing on the cave wall showed.
Mala initially mistook the drawing as a warning against the Fire Terrors and thought they were trying to harm the egg. However Hiccup noticed their behavior "did not seem predatory" and ultimately it is revealed that the Terrors were actually trying to defend and protect it.
It is possible that Hotshot is actually a Fire Terror, due to his red coloration.