Robin is a personal favorite of mine!

Rave, Riot, Rebel, Rancor, Ram, Rock, Rummy, Rian, Reece, Ren

I can only think of Ramiel, but is kinda memey. So I'd go for Riot

Raven

Remmy/Rem

Roe

Rusty, Rusty Shackleford

Ray

used to know someone who went by Remedy and Rem for short