Why is there is a human in cave on my embark area?

I've just embarked near a mountain, and on checking out my local area (3x3) I've noticed that there is a human in a small cave...

Is this normal? I've never had this before! I've not even unpaused yet, I wonder what will happen? Strike the earth!

He comes up as a "Current Resident". What is that?!

Update: Things have taken a turn for the worse.

Gods be damned, I'm only a week in. We're doomed.

Certain Nightcreatures (The randomly generated Night Trolls and Were-people), kobold clans, and semi-megabeasts claim caves during Legends mode, which have very real positions in the world.

Should you happen to accidentally embark on an occupied cave in Dwarf mode, you run into the scenario described above -- your starting 7 versus the "current resident(s)".

As you've already confirmed, when a single "civilized" race unit is the only occupant, it is because they are a Were-something, and will soon transform and cause havoc.

Vampires will not claim caves, Necromancers build towers (visible on embark select and usually non-embarkable), and Night Trolls self-identify with their current race, not whatever race the "Skulking Hag Groom" (etc.) originally was.

According to the wiki, Current Resident is normally applied to creatures that have killed a humanoid and thus have received a name. There's no mention of this applying to anything other than animals/monsters, so thiis might be a glitch, or a rare exception. On the other hand, there are reports of vampires as current residents, so what you may be looking at is a human vampire or werecreature that you just haven't identified as such... yet. Be careful.

(Edit: I didn't see your update until I was finished answering, so this seems to be confirmed...)