Xerneas does not have a Mega Evolution.
Lapras cannot evolve any further in Pokemon, nor does it have a Mega Evolution.
Mewtwo can Mega Evolve into either Mega Mewtwo Y or Mega Mewtwo X.
Aerodactyl can only Mega Evolve into Mega Aerodactyl. Permanently, Aerodactyl cannot evolve any further.
Do not give it to your Pokemon to hold, use it on your Pokemon like you would a potion.
In Pokémon Indigo, a Blastoisinite is used to Mega Evolve your Blastoise during battle. To do this, you must first ensure that you have a Blastoise that can Mega Evolve and that you have the Blastoisinite item in your inventory. During your turn in battle, select the option to Mega Evolve, which will transform your Blastoise into Mega Blastoise, enhancing its stats and changing its appearance. Remember, you can only Mega Evolve one Pokémon per battle.
Have it hold the Sceptilite, then press "Mega Evolution" during a battle to Mega Evolve it.
Pinser in Pokemon does not evolve into any other creature. Pinser does evolve into a Mega Pokemon using the Mega Stone at any level.
Mewtwo is the ultimate Psychic type legendary Pokemon. It was introduced in the first generation and does not evolve into any other Pokemon, although as of the sixth generation it can mega evolve into Mega Mewtwo X or Mega Mewtwo Y.
No, because Mega Evolution wasn't introduced until Pokemon X and Y.
Lapras cannot evolve any further in Pokemon, nor does it have a Mega Evolution.
Mewtwo can Mega Evolve into either Mega Mewtwo Y or Mega Mewtwo X.
Not to a normal evolution, but Pokemon that can do so can Mega Evolve during battles.
Aerodactyl can only Mega Evolve into Mega Aerodactyl. Permanently, Aerodactyl cannot evolve any further.
Hitmonlee cannot evolve any further, nor does it have a Mega Evolution form.
It doesn't evolve normally. If you have it hold a Tyranitarite, though, it can Mega Evolve during battle.
Onix evolves into Steelix, while holding a Metal Coat and traded. Steelix can also Mega Evolve, but cannot evolve into any other Pokemon.
Do not give it to your Pokemon to hold, use it on your Pokemon like you would a potion.