Like the above folks have said, it means 'make your own'! It's usually used in closed species, ARPGs, and sometimes user-made adoptable communities. The only people who can make characters for those two communities without an MYO are usually owners or artists for the communities, so most other people that want to create their own character for that species need to get an MYO ticket/item, usually through events, trading, or purchasing.
(I'm not really a member of any communities that make use of MYOs so take what I said with a grain of salt, but that's the jist of it from what I've gathered!)