Fire Vipers are colossal, snake-like monsters in the Skylanders series. For thousands of years, they are the preferred minions of Dark Portal Masters everywhere alongside the Hydra, the Sand Serpent, the Two-Headed Spider, and the less popular One-Legged Spider. They made their debut in Skylanders: Swap Force.
Fire Vipers possess the ability to breathe fire and blast molten spiked fireballs. They also hold the ability to fly. Their main weakness are the large Evilizer Crystals that grow on their heads and from inside their bodies that appear to be their life source.
A Fire Viper was mentioned in Wallop's backstory when it woke up from its slumber in the volcano Mount Scorch. The serpent then proceeded to attack the Trap Master's village until Wallop defeated the Fire Viper.
A Fire Viper makes a cameo in Stump Smash Crosses the Bone Dragon on the cover of one of Hugo's books entitled 'The Spotters' Guide to Hideous and Dangerous Monstrosities'. However, it is depicted as a regular snake with small horns on its head.
Despite them having Petrified Darkness on their heads, they are not evilized.
The Strata artbook for Skylanders: Swap Force shows that at some point Fire Vipers were an evilized creature created from a Crystal Master, but their original appearance was not depicted.
↑Skylanders: Stump Smash Crosses the Bone Dragon, page 70