Darkmist Widow can be found with NPCScan, but it isn't really necessary as she only spawns in the one location. If you do choose to add it, her NPCScan ID is 4380.
Before Cataclysm her spawn timer was somewhere between 5-8 hours. I am fairly certain that it has since been significantly reduced. I came across her on only my second time in the area.
There are a total of five rare tamable hunter pets in Dustwallow Marsh. All of them are:
- Darkmist Widow, level 37 black & red spider
- Dart, level 37 orange raptor
- Hayoc, level 37 white wind serpent
- Oozeworm, level 38 yellow worm
- Ripscale, level 39 crocolisk