In the U.S. TV series "Game of Thrones", what do the Dothraki fear the most?

In the U.S. TV series "Game of Thrones", the item which is feared the most by the Dothraki is sea water.

The Dothraki refer to the ocean (the saltwater environment) as the poison water. They dislike all saltwater seas; they mistrust and fear them. Any water that a horse cannot drink is something foul. All the heaving great grey-green plains of the ocean fills the race of nomadic horse-mounted warriors with superstitious loathing and great wariness. Additionally the Dothraki do not like the fact that a horse cannot cross the poison water on its own legs. This is a bad sign which sane and prudent people have to find threatening.

The Dothraki believed that the earth ended at the great salt sea. Prior to his vow to take the Iron Throne for his unborn son, the idea of crossing the black salt sea gave even Khal Drogo great unease and pause. He knew that other people used ships on the ocean. The Dothraki, however, referred to ships as wooden horses. The fear triggered by the specific stimulus of not knowing what mysteries might be in the sea or ocean could never be ignored even if or when the Dothraki would use wooden horses. This type of fact situation could only cause great problems for all concerned.