Fyrnocks were silicon-based creatures originally indigenous to the jungles of the planet Anaxes. Surviving the destruction of their homeworld, many fyrnocks inhabited the resulting Anaxes asteroid belt, including the asteroid PM-1203, which contained Fort Anaxes, a military base used by the Grand Army of the Republic during the Clone Wars. Fyrnocks were hurt by sunlight and could only live under the cover of darkness, in which they became fierce predators. However, they were immune to lights that were not solar-based.
At least one fyrnock was known to be many times the size of standard fyrnocks. This creature was far more dangerous than standard fyrnocks, being, in fact, the mother of all fyrnocks. Years after the Clone Wars, Jedi Padawan Ezra Bridger summoned this fyrnock, but had to tap into the power of the dark side to do so. The fyrnock put up a vicious fight against the Grand Inquisitor, buying Ezra and Kanan Jarrus necessary time to escape, but Ezra was briefly knocked unconscious and left shivering by the unexpected power of the dark side energy that he had summoned.
A large group of fyrnocks being tamed by Ezra Bridger
During the Battle of Anaxes, fyrnocks prowled around a campfire set up by Clone Force 99, CT-7567 "Rex," CC-2224 "Cody," CT-5597 "Jesse," and CT-6116 "Kix." Many years after the Clone Wars and the planet's destruction, Hera Syndulla and Sabine Wren encountered fyrnocks in the abandoned Fort Anaxes. Later, Jedi Knight Kanan Jarrus and his apprentice Ezra Bridger returned to Fort Anaxes, hoping to lure the Grand Inquisitor into a trap. Using the Force, they were able to convince the fyrnocks to attack the Inquisitor and his Imperial stormtroopers. While the fyrnocks were ultimately unable to defeat the Inquisitor, they did manage to take down several of the stormtroopers.
Fyrnocks first appeared in the Star Wars Rebels episode "Out of Darkness." According to the episode guide, they were originally to be named garcors, and then nockfyrs in the manner of bats. Series executive producer Dave Filoni suggested that the name's syllables be reversed to "fyrnock."
It is strongly implied in the episode guide for "Gathering Forces" that the powerful fyrnock summoned by Ezra is, in fact, the creatures' mother. This was confirmed in the title Ezra's Duel with Danger.