Denver Nuggets Mascot -- PASSES OUT ... While Being Lowered From Rafters

Denver Nuggets mascot Rocky the Mountain Lion's motionless body was lowered from the Pepsi Center ceiling last night before their home opener ... but the big mystery is why.

The plan called for Rocky to be lowered from the rafters with a spotlight on him ... but when his descent to the floor began -- Rocky didn't move a muscle ... and instantly collapsed when he touched the ground.

According to reports ... the guy in the suit was checked out right after and walked off the court on his own, but so far ... the team has been hush-hush on what happened.

Good news -- the team should have Rocky back for their next game.