chǒu niú

On its own, the leading character 丑 means ugly. As part of the twelve counting characters though, I have been told that it has no particular meaning. Still, it doesn't do wonders for a young girl to be told that her zodiac sign is an ugly cow.

The character 牛 is gender neutral. It can be used for any sex and, I believe, any .. uh .. fixed-status[1]. Hence its translation to ox in the zodiac. 牛 is a very commonly used word when referring to that class of animals, much more common than "ox".

[1] So I tried looking up the Chinese word for a fixed male ox, but steer is such an overloaded term that I could not find a proper translation. However, if it follows the general rule of thumb, it would translate into something along the lines of [description of being fixed]牛. Probably.