If a card in a player's graveyard has {X} in its mana cost, X is considered to be 0.

The set of creatures affected by Chittering Host's triggered ability is determined as the ability resolves. Creatures you begin to control later in the turn won't get +1/+0 or gain menace.

For more information on meld cards, see the Eldritch Moon mechanics article (https://magic.wizards.com/en/articles/archive/feature/eldritch-moon-mechanics-2016-06-27).