Fresh and decayed corpses are those that still have flesh upon them. If their decay is advanced, or if you wish a skeletal servant instead, place the corpse along a coast or in a swamp or marsh. Animals are the Necromancer's greatest allies when it comes to stripping the flesh from a corpse. The ravenous mudcrabs of Morrowind can strip a corpse down to its bones in a matter of days. Lesser crabs in other provinces can do the same in a matter of weeks.