In the dictionary, the word noctivagant appears like this:
noctivagant: adj. Going about in the night; night-wandering. [L. nox, noctis, night + vagans, p. pr. of vagari to wander about.]
So in essence, this would be a creature that evokes the description of "The Night Wanderer".
Intro Level Creature Idea
Assuming that this would be for a low-level party ranging from levels 1-3, this could be a kind of undead. When encountered, this humanoid creature appears to walking deliberately to some unknown destination. Upon closer inspection, the creature eyes are closed. It does not respond to any normals attempts to communicate. The special ability would be a sleepwalking effect on all creatures that come within 60 feet that fail a saving throw. The sleepwalking effect would basically cause those affected to have all the effects of a sleep spell, but would also be compelled to walk slowly in a random direction while within 60 feet of the noctivagant or 10 rounds, whichever is greater. Under the influence of the sleepwalking ability, an affected creature can walk into trees, step off cliffs, etc. They cannot, however, climb or perform any other action other than walking. If a creature dies under the influence of a noctivagant, they become one. Once they become a noctivagant, they are no longer in danger of walking into trees or off cliffs, etc.
Being attacked breaks the effect of the sleepwalking ability.
In the end, this is like more of a nuisance than threat.
A Devious GM
However, take one of these and waltz them into a village and now the character cannot tell which is the noctivagant and which are the victims of the attack. Characters that favor combat as a solution will end up creating more of these things. A clever PC may attempt a "Talk to Dead" spell. Victims of the sleepwalking ability aren't dead, so will not respond. Still, the PC may not be close enough to the noctivagant to get the desired effect.
Come to think of it, these things could really wreak havoc. Get an elf (immune to sleep) to collect one of these and unleash it into huge crowds. (Who says there's no such thing as an elven jester?) Better yet, an adventurer could take one of these and turn it into a necromancer's idea of a ten foot pole. Why poke and prod looking for traps when this thing, which is already dead, can just walk in and trigger everything.
I could see this as something a necromancer initiate would create. One of these could certainly disperse the "peasant angry mob". It could even divert small armies away.
Stats coming later. Nothing more than 2 Hit Dice.