Why Derrick Rose loves his ‘unicorn’ so much

Derrick Rose and his precious "unicorn" medicine ballThere’s more than one “unicorn’’ in the Knicks’ locker room this season. Kristaps Porzingis was dubbed a unicorn last season by Kevin Durant, but Derrick Rose has brought a second one with him.

It’s Rose’s trusty medicine ball, which he carries everywhere and nearly lost — twice — when he was in Chicago last Friday. Rose, who has had the ball since the 2010-2011 season with the Bulls, said the heavy ball is a perfect weight, which he has been unable to find anywhere else.

“It’s the only ball that’s like this in the U.S.,’’ Rose said. “It’s a 4.5-pound ball. Every other ball is three or six. No one else can find it. I’ve got a unicorn right…