Yeah, don't use a 6V horn on a 12V system. You get double the current and the horn will burn up pretty quick (after you put a large enough fuse on it!)

Yes you could use a voltage divider (reducer). Some of them are clunky; shop around some for your best selection. Make sure you are using a relay with the horn as they tend to be fairly snappy with the current draw. Modern fuse blocks usually have one built in standard for the horn.

Regards, Kitz