>>38801997Megas were essentially an additional evolution for certain pokemon, but instead of evolving by normal means you could only do it in battle, and then they would revert back after the battle. The pokemon has to hold a certain item to mega evolve, and you can only mega evolve one thing per battle. They could get new abilities, typings, and stat arrangements that made it play completely differently from the base form or simply breathe new life into obscure shitty mons like Mawile.
Dynamaxing is also one time per battle, and from what we know it just makes your pokemon stronger for 3 turns. This is pretty much the same thing as Z moves, which are yet another one time per battle thing that boosts your pokemons move for one turn.