True Alpha describes the method by which a werewolf can attain the status of alpha werewolf without having to kill and take the status or inherit the status from another alpha.
According to Dr. Deaton, the process by which a werewolf rises in status to alpha due to their own willpower and strength of character is rare.
- “It's rare, it's something that doesn't happen within 100 years, but every once in a while, a beta can become an alpha without having to steal or take that power. They call it a true alpha. It's one who rises purely on the strength of the character, by virtue, by sheer force of will.” — Alan Deaton to Scott in Currents
According to Teen Wolf creator Jeff Davis, the term "true alpha" only applies to how one attains the status of alpha. Once an individual becomes an alpha this way, they can't lose the status unless they die or choose to give it up to heal someone (see Derek's sacrifice in Alpha Pact).
The only residual quirk of the true alpha method is that your status cannot be taken except by a beta that you created. In the case of Scott McCall, he has two bitten betas that could possibly take his status.
According to Deaton, the true alpha method hasn't occurred in his lifetime. This suggests that, besides Scott, no other alpha werewolves mentioned or seen on the show could have risen to the status on their own.