Since I thought of him as a Mountain Stingtooth, it was necessary to find him the appropriate Flyer form to match. The species that fit very well was Anhanguera (which belongs to the same family as Sierra’s species - Anhangueridae).
He got his name for his incomplete tail.

Out of the entire pack, he can be considered the most mysterious of the four. In fact, Tailless' story goes much deeper and further back before he joined Silverwing's pack.

Once upon a time, Tailless had a family: A mate and a young son. His native flock settled in the shadow of the Black Mountain of the Forbidden Lands. They peacefully coexisted with other species, until one day the Flyers invaded their home. The invaders had killed most of the Stingteeth, leaving only a few of the hatchlings alive. The Flyers used them to put them against the young of other Backwing species, especially the Softheads.

The leader of the Flyer flock, Cerada, had a keen eye on Tailless, as even with the massacure happening around him, he stood strong, fighting off the flyers. He was powerful, skilled and armored. Very suitable qualities for a warrior under her command. Thus, she offered him a deal he could not refuse: She would leave his offspring alive if he agreed to serve her and guard the mountain from uninvited guests. Tailless didn't have a choice, as the parental connection with his offspring was too great, and, should he lose the youngling, the parents sometimes lose the will to live.

From that point on, Tailless became the guard of the dungeon caves. Occasionally, he was allowed to see his son so that he could make sure that his young one was alive and continue his service. But over the course of his life side by side with the Flyers, he had become more silent and more bloodthirsty. Cerada didn't hesitate to feed him young Softheads that rebelled or tried to escape. Besides, Thrasher liked to get into a fight with Stingtooth. He considered it a good exercise in his skills, and it was also a way of showing the Stingtooth his place. Tailless knew that the reign of this pack would be long, but continued to serve them for one thing - the life of his son.

Despite the fact that the Flyers trained young Stingteeth to fight, Tailless’ young one (who has not yet been named) made friends with some of the other children. One such friend was Viper, a female Wood Stingtooth. Through her, he met Draigar, who suddenly managed to enter the mountain. During the escape staged by the red backwing, Tailless’ young one had died while protecting his friends. Cerada did not tell Tailless anything, not wanting to lose her loyal servant and enforcer. However, due to the lack of meetings with his son, Tailless was tormented by doubt and gradually he became more uncontrollable.

It was then that Cerada put forward a condition for him: Tailless would go in the guise of a Flyer along with Silverwing’s flock and bring back the Red Crystal. Once the Backwing was delivered, she would let him go from her service along with his son. In reality, she didn't plan to honor her end of the bargain.

In the Great Valley, the False Flyer had a very difficult time: he had to get used to his new form with wings, along with new creatures with new smells that were everywhere, in addition, some females also laid eyes on him. Tailless closed himself off from everyone, including his new flock. He didn’t want to join them, although Silverwing was kind enough to insist that Tailless do join them. However, Tailless had to accept the fact that at least, now he was not alone and did not count his days down in a gloomy dungeon.

When Sierra fell into their clutches, Tailles mistook him for Thrasher, since that Flyer was so similar to him. The False Flyer took out all of his anger on an innocent captive, not skimping on using his teeth and claws.

Everything changed during the Battle of Threehorn Peak. Tailless was assigned to guard the Draigar, who prevented the gang from killing Sierra. Draigar, recognizing Tailless as the Stingtooth of the Black Mountain. She then told him the sad fate of his son. That, coupled with the red flags he had before, he realized that Cerada was deceiving him. Deciding to make amends and write the wrongs he had done, Tailless attacked Silverwing, damaging the membrane of his wings, thus, giving Thunderstorm (Sierra in Backwing form) an advantage in battle. Unfortunately, his betrayal of Silverwing’s flock had cost him his life, as he threw himself and Sharppeck off a cliff. Hopefully his sacrifice would allow him to reunite with his son in the Great Beyond.