When you cast a creature spell for its mutate cost, a few things change about how you cast it. First, you pay the mutate cost rather than the mana cost. Second, the spell will require a target. Specifically, a target non-Human creature you own. Humans on Ikoria spend all their energy merely surviving, so their cells just aren't up for the mutation thing, I'm guessing. Mutate can be used only if you're casting the spell. If a creature card with mutate enters the battlefield some other way, it just enters the battlefield.

You can have up to one chosen companion for each game. That chosen companion doesn't start in your main deck. Rather, it's a card in your sideboard. This means it doesn't count as a card toward meeting the minimum deck size in the format you're playing, but in Constructed formats, it is one of your fifteen sideboard cards. Just before the game begins, reveal your chosen companion to all players. Once during the game, but only during your main phase if the stack is empty, you can pay {3} to put your chosen companion into your hand.

Among the strange and unusual wildlife on Ikoria, five specimens are awe-inspiring enough to feature their own myths and legends. Each card in the Mythos cycle can be cast with a single color of mana, but the tale is much more epic if you also spend a certain two other colors of mana.