Long Hair is rare in the NBA. But on the Boston Celtics there is
Kelly Olynyk who sports long hair.
What's also interesting is the team he's playing for. I think Celtic men were also known to have long hair in history.
I believe you're thinking of the 2004 Boston Red Sox rather than the Celtics. There were very few long haired Celtic players.
An update on this: to make cap room space Kelly Olynyk the Celtics ronounced their rights to him. He signed with the Miami Heat.
He became became an unrestricted free agent after Gordon Hayward was signed. (The Celtics had to clear cap space.)