Florida Panthers -- Stanley Panther Sues ... How Could You Fire a Depressed Mascot???

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.

Estevez says he was removed from the Panthers "Energy Team" -- and was later fired in April.

In the lawsuit, Estevez says he believes he was fired for his disability -- essentially saying the team didn't want a depressed guy in charge of pumping up the crowd.