Alatatalantecuimar or Artalantecuimar(Q."Great scale-Animals"; "Noble scale-animals";"Dinosaurs") once roamed Arda when the world was lit by the two lamps and Arda was one. Almaren and Utumno stood polar opposite to each other as the Giant Lizards and Nameless did to each other. It was the Altalantecuimar that intrigued Morgoth and it was these creatures that he eventually used to create the dragon and fell beast races.
Though some Maia and some Spirits of Arda watched over these powerful creatures--the majority were wiped off of Arda when Melkor came out in war and destroyed the Two Lamps. The ensuing fires engulfed and destroyed the majority of all Giant scale-beast species--so only few of these creatures remained. The Altalantecuimar that remained either avoided the flames totally or became lesser in stature. Eventually Melkor would sacrifice Maia spirits in order to create his own perverse versions of these creatures, and they would become powerful units in his armies.
When asked about the nature of the "steed of the Witch-king", Tolkien replied that the fell beast was not intended to be pterodactylic, but hesitantly acknowledged that it resembles a pterosaur and may have been a survivor of older geological eras.Thus the "Altalantecuimar" were introduced as a possible family of animals from these unknown "older geological eras" which might have been the ancestors of these Fell Beasts and probably the Unbirds, Carchadils and Assangar as well.