>>41561244According to Gen 6 promotional material about Mega Gardevoir, the horn is a "manifestation of this Pokemons heart".
I was wondering about official context on it a while back and did some digging. The quote is on a few wikis and blogs, but no official site has it anymore. You can find the quote here if you go to this page and then click Gardevoir / Mega Gardevoir at the bottom.
https://web.archive.org/web/20150207083705/http://www.pokemonxy.com/en-us/pokemon/mega_pokemon/>The red plate in Mega Gardevoir's chest is said to be a physical manifestation of its heart. Mega Evolving enhances Gardevoir's Sp. Atk significantly.Also judging by Pokken's representation of it and it officially being referred to as a "plate" here, I'd assume it's very sharp and solid to the touch, more bone-like than flesh-like.
