A ewe is a female sheep. Since the word is pronounced 'you', there is some confusion surrounding this word. Strictly from spelling 'an ewe' seems more appropriate, but from a pronunciation point of view, 'a ewe' is superior. While I'm not sure which version is more gramatically correct, a google search suggests that 'a ewe' is more common (oddly, some websites use both methods of spelling), so I'm inclined to keep it at 'a ewe'.