You must choose an existing creature type, such as Squirrel or Knight. You can't choose card types (e.g., artifact) or supertypes (e.g., snow).

You choose the target player as you cast Pact of the Serpent, but you don't choose the creature type until the spell resolves.

The value of X is based on the number of creatures the player controls of the chosen type as Pact of the Serpent resolves.