Latin had three genders. Most modern Romance languages had lost their neuter gender, because they've merged with the masculine gender.

Also masculine and feminine genders have merged into a common gender in Dutch and many Scandinavian languages. And English had lost them all. (He/she/it distinction is a relic of that lost feature)

These are mostly caused by phonetic changes. If two different genders have similiarly pronounced declensions, they eventually become merged.

I don't know any added genders though. The most similar example would be loaned genders, which only work for loanwords. (E.g. actor-artist, blond-blonde) These words are used for different gendered persons, but they still won't be considered as true genders.

Please correct me if I have any mistakes, though. I'm not a linguist.