Where can I play an online version of Pokemon Emerald that I can alter?

Ok so one day i saw a youtube video of a walkthrough for Pokemon Fire Red/ Leaf Green Version. i watched just one episode of it and discovered it was a hacked play because the guy had Pokemon you couldnt get yet in the areas he was at (he caught a Nidoran male in route 2 right before he got into Pewter City which is impossible). When he battled Brock, the gym leader also had a lvl15 Tyranitar! I also saw at the end of the video (or in the comments cant remember which) he asked the viewers what pokemon he should catch at mt.moon and said he was thinking of getting larvitar! all this amazed me but i just closed the video without giving it much thought but now im thinking i would love to try that for Pokemon Emerald Version. only thing is i dont remember who the video i watched was by and cant find it and i dont know where to look for it. so im asking you lovely people if anybody knows where i can find an online version of Pokemon Emerald that i can change things such as the pokemon you can find in certain routes and gym leaders pokemon. im not really interested in changing the map layout or anything i just want to be able to, for example, catch a bagon in route 101 :D i also want to change up the gym leaders pokemon teams so they'll be harder to beat (your gonna want a challenge if you have salamence by the time you battle your dad :P) so thanks everybody who helps me out on this!!!!