Beta Megas 

Several concepts for Mega Venusaur, depicting it much chunkier and larger than it ended up being, as well as looking quite different from its normal form altogether. Some designs had a few different colorations hidden within the image files.

Mega Blastoise has several concepts that depict it, again, much chunkier than the final and its normal form. It also appears with a bushy mustache and tail, similar to Wartortle's ears and tail. These features are reminiscent of MissingNo Index 181, Wartortle's original evolution from Pokémon Red and Green's prototype stages, which Blastoise replaced. Given this information, it seems this design was trying to call back to that design and marry the two concepts.

Several different designs for Mega Charizard X/Y, most of which come in multiple color revisions. Unlike the previous two starters, these resemble the final designs a little bit more closely, but were still changed up a fair bit for release.

A couple different concepts for Mega Mewtwo Y, with a few different color variations.

Mega Jynx is the only scrapped Mega Evolution that was planned for Pokémon X & Y. This Mega Evolution was scrapped out of fear of any possible controversies for Jynx appearing.