Kotis, the Fangkeeper

If an exiled card has {X} in its mana cost, X is 0 for the purpose of determining its mana value.

If any of the spells you cast has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Since you are using an alternative cost to cast the spells, you can’t pay any other alternative costs. You can, however, pay additional costs, such as kicker costs. If the spells have any mandatory additional costs, you must pay those.

You cast the spells from among the exiled cards while Kotis’s last ability is resolving and still on the stack. You can’t wait to cast them later in the turn. Timing restrictions based on the card’s type are ignored.