If the copied creature card has in its mana cost, X is 0.

If Feldon's ability creates multiple tokens due to a replacement effect (such as the one Doubling Season creates), each of those tokens will gain haste and you'll sacrifice each of them.

The token copies exactly what was printed on the original creature card (except that the copy is also an artifact) and nothing else.

If another creature becomes a copy of, or enters the battlefield as a copy of the token, that creature will copy the creature card the token is copying, except it will also be an artifact. This is true even if the creature card that was copied by Feldon of the Third Path is no longer in the graveyard at that time. However, the new copy won't gain haste and you won't sacrifice it at the beginning of the next end step.

Any enters-the-battlefield abilities of the copied creature card will trigger when the token enters the battlefield. Any "as [this permanent] enters the battlefield" or "[this permanent] enters the battlefield with" abilities of the copied creature card will also work.