Nightmare endo is the animatronic endoskeleton that was used as an inner skeleton for Nightmare Fredbear and Nightmare. The endoskeleton first(though obscurely) debuts in five night's at freddy's 4, and returned as a separated minor character in five night's at freddy's AR  help wanted. 

Physical apperance:

Nightmare endo is the endoskeleton of nightmare and nightmare fredbear as can be understood from his apperance and movements. He is a tall and massive skeleton with huge curved claws and razor sharp teeth. Unlike Endo-01 and endo-02, he has very thick, complex limbs, specially extended in order to steadily hold a bulky body. He has five long, thumbs on his clawed hands like other nightmarish animatronics. On his back area is a small steel vertebrae soldered in a line. The nightmare enderskeleton has an elongated head with a large mouth, with short but sharp white teeth set apart. On top of its head are two frames of large bear ears. The endoskeleton has glowing red eyes with metal lenses, rather small in relation to its build.

Functionally:

While bearing some functional movements like most endoskeletons, nightmare endo's sharp teeth and claws are presumably meant to be used for slaughtering victims.