Which compound is known for having a fecal odor and is represented by the formula C9H9N?

Skatole, represented by the formula C9H9N, is a compound well-known for its strong fecal odor. It is a byproduct of the decomposition of amino acids in the digestive system of animals and is particularly associated with the breakdown of the amino acid tryptophan. Skatole is found in feces and is primarily responsible for the characteristic scent associated with them.

Indole, while chemically related and also having a fecal odor, is distinct from skatole; indole has the formula C8H7N and is less potent in its fecal scent. Mercaptans, known for their foul odor, are sulfur-containing compounds and are not represented by the same formula; they are often associated with gas leaks and not primarily with fecal sources. Ammonium itself refers to the ammonium ion (NH4+) and does not have the specified odor associated with feces, although ammonia can form from the breakdown of urea in urine.

Hence, skatole is specifically recognized for its fecal odor among the choices provided, making it the correct answer.