I think this show was on Disney XD, Cartoon Network, or Nickelodeon but I didn't find it on the list of shows. I saw it sometime between 2009 and 2013. Don't remember too many details and I don't know if things are blending together from other shows. It was an action type of show, but not anime style.
There was a guy in high school and I think he was either hiding that he could transform into a creature or he got his "powers" then. I feel like he might have drank something like orange juice but it actually wasn't.
He ended up fighting another dude in his high school who was the same thing as him. He might have grown a tail when he changed and he could change into the creature whenever he wanted to. I think the fight happened at the high school.
I'm super frustrated because I can't remember the name of it and have searched on IMDb and other platforms and haven't found anything. I know I would recognize it if I see it.