"The eighth horse does not have water."

Translation: Al optulea cal nu are apă.

[bluchill]

A optea cal nu are apa?

[potestasity]

Ordinal numerals change by gender. Here, it's either "a opta" (feminine) or "al optulea" (masculine); the word "cal" is strictly masculine; you can use "iapă" (mare) instead: "a opta iapă nu are apă" (hey, it rhymes!).