I was doing the magikarp challenge where you have to evolve it to a Gyarados, after I did the challenge, I released the Gyarados but it had the exp share on it. Is there anyway I could get it back?
Unfortunately no, you cannot receive your exp.share back since you released your Pokemon with it. The only way you can get one back is if you buy one off of the server store or, trade another player for one. Please be more careful when you release your Pokemon and, make sure there are no items you want on them when you do so!