Nightmare Child

Nightmare Child was a phenomenon or entity that was extant during the first year of the Time War. Davros' command ship flew into the "jaws" of the Child. Despite the war being Time locked, Dalek Caan managed to enter the war with an emergency temporal shift. (DW: Evolution of the Daleks) He arrived in the middle of the battle and saved Davros. (DW: The Stolen Earth)

In the End of Time part 2 the Doctor said that Rassillon's plan to return Gallifrey would lead to the Nightmare Child (And all the other monstrosities of the Time War) being pulled out of the Time War and in to this reality. This seems to indicate the Child might actually be a physical entity,

The Tenth Doctor states "I saw Arcadia destroyed. I laughed in the face of the Nightmare Child." after reliving a memory from his Eighth Incarnation. He makes no reference as to what the Nightmare Child may be. (IDW: The Forgotten)

It has not yet been precisely explained what a/the "Nightmare Child" actually was.