Remember, damage dealt to a player causes that player to lose that much life (unless the source has infect).

If no opponent has lost life on the turn Cryptborn Horror enters, it will enter with no +1/+1 counters on it and be put into its owner's graveyard (unless something else is raising its toughness).

Cryptborn Horror cares about the total life lost, not necessarily what an opponent's life total is compared to what it was at the beginning of the turn. For example, if an opponent loses 5 life and then gains 10 life on the turn Cryptborn Horror enters, it will enter with five +1/+1 counters on it.

In multiplayer games, life lost the turn Cryptborn Horror enters by opponents who left the game later that turn will count toward the value of X.