"Who is standing in front of the door? Peter something."
Translation:Ki áll az ajtó előtt? Valamilyen Péter.
We don't know Péter's last name. Which, in Hungarian, is not the last name. It comes first. A person's name in Hungarian is family name first, given name next.
So, the speaker here understood the person's given name, "Péter", but forgot his family name. So he or she says "Valamilyen Péter". Or, in English: "Péter something".
I see, OK. I think that difference comes from the position of the family name. It comes first in Hungarian. And if we think of the given name as a noun (which it actually is: a proper noun), well, then the family name is in perfect position to act as an adjective, modifying that proper noun. Many times it actually is, or sounds like, an adjective. Let's pick a name: Fehér Péter (translated: Peter White, that is, White Peter). Can you see it now? It comes very naturally to refer to that unknown family name as "valamilyen". Which would not be possible if the family name were last.
Naturally, in the opposite case, if the given name were not understood, then the reference would be most probably "valami"/"valaki"/"akárki", basically, somebody or whoever: "Fehér akárki".
If you say "valami Péter", that version is also possible but it is a little bit different. It does not necessarily refer to Péter's family name. Rather, it would be something like "some Peter" or "a Peter" or "a certain Peter" or "some kind of Peter".
Hungarians say, "Some kind of Peter", whereas English speakers say, "Peter something"