Fierceclaws was a female NightWing-IceWing hybrid who was mentioned in Darkstalker. She was one of the six possible dragonets that Clearsight and Darkstalker might have had in a formerly possible future.
Fierceclaws never existed in the timeline the books take place in.
The Jade Mountain Prophecy
Darkstalker mentioned that he and Clearsight would have had six dragonets.
The Lost Continent Prophecy
Clearsight mentioned Fierceclaws indirectly in her thoughts when she thought that she would always miss the dragonets that she would have had with Darkstalker.
When Darkstalker gave Clearsight the mind reader-blocking bracelet, she said to him that Eclipse would need it most. This led to their first conversation about their dragonets. Darkstalker listed their three future dragonets, Eclipse, Shadowhunter, and Fierceclaws. Clearsight told him that she would not let the dragonet be named Fierceclaws, which then led into a friendly debate in which Darkstalker argued that the name Fierceclaws was cool. Clearsight disliked it. He then suggested that they name their daughter Foeslayer, after his mother.
Later in the book, right before Clearsight put Darkstalker into eternal sleep, he mentioned to Clearsight that he had foreseen that he would never manage to convince her to use the name Fierceclaws for their dragonet.
= Possible timeline