Beast Boy’s first appearance was Doom Patrol #99 released in 1965. Beast Boy’s real name is Garfield Logan. He has been on The Teen Titans, Doom Patrol, Young Justice, and the Ravengers.  

(Garfield’s Comic Origin) Garfield Logan was merely a baby when he contracted the extremely deadly tropical disease sakutia. His parents being scientists doing research in West Africa they tried to figure out how to save him. They noticed that the only organism that survived the disease was a green monkey. Mark Logan, Garfield’s father put him through experimental a ray treatment. This amazingly made Garfield go backwards through the evolutionary chain. Which cured him and turned him green soon after this Gar discovered he could transform into many different animals. Months after discovering this his parents and him were canoeing down a river when they ran off a waterfall. Garfield’s ability saved his life but not his parents and Garfield was alone. He ended up working with the Doom Patrol ,but left due to them being too brutal.  

Garfield Logan can turn into any real animal not Being able to turn into mythical beast of fictional monsters.