Lv50 creature in lower level area?

[Bixby]

Hunt marks are also tougher than regular level 50 mobs. They're elite, with significantly more HP than their level alone would indicate. When you see the icon is shaped like a starburst or some other fancy thing instead of the typical alien face (non-aggressive) or alien face in a triangle (aggressive), you know it's going to be (comparative) trouble.

[Erudain]

Even if you can solo the B-rank (the non-aggresive ones) you'll get nothing....maybe some exp (they give exp to Chocos).
You can't get the money and seal reward unless you have the hunting bill for said mob.

A-ranks and S-ranks cannot be soloed, you'll need several ppl helping.

[Alukah]

http://ffxiv.gamerescape.com/wiki/The_Hunt

Don't try to solo A ranks or S ranks, B ranks are worthless unless you get the weekly bill from your GC (once you hit 50), when A or S rank spawns you will most likely see a flood of shouts of people asking for invite, the best way to get full rewards from those monsters is by partying up with other people.

[Enkidoh]

http://ffxiv.gamerescape.com/wiki/The_Hunt

Don't try to solo A ranks or S ranks, B ranks are worthless unless you get the weekly bill from your GC (once you hit 50), when A or S rank spawns you will most likely see a flood of shouts of people asking for invite, the best way to get full rewards from those monsters is by partying up with other people.

They're not entirely worthless, as they still count towards the Hunt achievements regardless if you have the Hunt Bill or not, so if you're after the quest titles from them it's still worth doing. Or if you're bored and are feeling particularly bloodthirsty (I like to hunt B rank NMs for sport myself even if I don't have the Hunt Bill for them).