What did mapmakers actually write on unknown territory?

Almost never here be dragons. The phrase appears on essentially one surviving artefact - the Hunt-Lenox Globe of about 1510, which carries HC SVNT DRACONES off the coast of Asia - and possibly one related egg-shell globe. That is the entire evidential basis for one of the most confidently repeated facts about old maps.

What cartographers actually wrote in blank spaces was usually terra incognita, or nothing, or decorative filler. Sea monsters were drawn frequently, and were not really warnings; they were partly ornament, partly a claim that the mapmaker knew what lived there, and partly a way of filling expensive vellum that would otherwise look unfinished.

There is also a well-known line about Africa attributed to Jonathan Swift, mocking geographers who filled gaps with elephants for want of towns. That is the honest version of the practice: the blank spaces were an embarrassment, and the standard response was to decorate rather than admit ignorance. Later mapmakers went the other way and left them empty, which was a genuine change in what a map was understood to be for.