Stanley Panther says he was BETRAYED by his own NHL team -- claiming he was unfairly fired as the team mascot after a serious bout with depression because the honchos didn't like his attitude.
The man behind the lawsuit is Raphael Estevez -- who says he was hired to play Stanley full-time in 2012 and everything was cheertastic until March 2016 ... when he was hospitalized for depression.
Estevez says he missed a week of work -- and when he came back, he was treated like an outcast ... and was falsely accused of taking money from fans and having a major attitude problem.