That's a hard question! When you are from somewhere you say Olen Suomesta kotoisin. Here's a few ideas! In old finnish the word "koto" was used instead of koti, where kotoisin could come from. The word "kotoinen" means "homely", so it might have something to do with that! In general the pair koti - kotoisin is very unique, I cant think of a single other word that changes the same way.