Human is a monster attribute that indicates that a monster should be considered human. It has many effects, but most significantly it helps determine whether your actions constitute cannibalism, same-race sacrifice, or murder.
The following 71 monsters have the human attribute, M2_HUMAN:
The human attribute is used to determine the following things:
- Whether eating a monster is cannibalism for a human player.
- Whether a sacrifice is of your own race for a human player.
- Whether killing a monster is murder. Killing humans (and only humans) generated peaceful is murder, no matter what your starting race.
- The outcome of an attempt to tame a monster - human monsters cannot be tamed.
- The monster a randomly generated figurine is for. Figurines of human monsters are (almost?) never generated.
- What to give when wishing for a figurine - wishing for a human monster gives a random figurine.
- What to give when wishing for the corpses of quest guardians
- What responses to use when chatting with other roles' quest guardians (possibly created by reverse genocide or found in bones).
- Whether a new form is appropriate when a monster is polymorphing 
- Whether monsters respect Elbereth or a scroll of scare monster when you are not standing on it - humans do not. As @ do not respect Elbereth at all, this applies to monsters of other classes with the human attribute, only - currently, this means K Keystone Kops.
- Whether to adjust your alignment record when a monster is genocided. Genociding human monsters reduces the alignment record of lawful adventurers, and increases that of chaotic ones.
- How shopkeepers address you (cosmetic only).
- Whether a werecreature is in human or animal form.
In the source, the human attribute is represented by the macro M2_HUMAN.
These strange creatures live mostly on the surface of the
earth, gathering together in societies of various forms, but
occasionally a stray will descend into the depths and commit
mayhem among the dungeon residents who, naturally, often
resent the intrusion of such beasts. They are capable of
using weapons and magic, and it is even rumored that the
Wizard of Yendor is a member of this species.
- ↑ 1.0 1.1 monflag.h, line 115 monster attribute M2_HUMAN
- ↑ eat.c, line 442
- ↑ pray.c, line 1167
- ↑ mon.c, line 1855
- ↑ dog.c, line 803
- ↑ mkobj.c, line 507
- ↑ objnam.c, line 2567
- ↑ objnam.c, line 2560
- ↑ sounds.c, line 439
- ↑ mon.c, line 2408
- ↑ monmove.c, line 899
- ↑ read.c, line 1636
- ↑ shk.c, line 2275
- ↑ were.c, line 16