Ahad is indeed a muslim name, however only if it is kept in the correct way. The meaning of Ahad is "The One and Only" therfore, quite obviously it cannot be used to name a human as only Allah swt can be referred to being one and only. However if the name is beginning with 'Abdul' meaning "servant of Allah swt" then the name will become acceptable i.e. Abdul Ahad. Allah Knows Best...