Otherworldly Phantasmal Horse. The third cheat Noble Phantasm used by Rider of “Black,” Astolfo. There are cases when a Griffin, a Monstrous Beast, reproduces with a mare, which would normally be its food. What's born as a result from this is the Monstrous Beast known as a Hippogriff, which has the upper body of an eagle and the lower body of a horse.
However, it is an impossible creature which normally should not exist. This is because it should be impossible for a child to be born between a Griffon and a horse, which are predator and prey. The Hippogriff existed only as a hypothetical and philosophical symbol. A Hipppogriff appears in the story of “Orlando Furioso” (The Frenzy of Roland), where Astolfo and his comrades also appear. This story includes the theme of “The Hippogriff makes an appearance here, so no matter how strange it may seem, there is nothing that is truly impossible.” In “Orlando Furioso,” Astolfo even goes to the moon in order to save Roland.
The Hippogriff that Astolfo rides is a “Phantasmal Beast” in the true sense, since its very existence is uncertain. As a result, when it is released as a Noble Phantasm, the Hippogriff can jump over to a different dimension, though only for an instant.
That dimension is the home of the Phantasmal Species, where only beings that are purely non-physical souls can go to. It is the Reverse Side of the World, which the Hippogriff, when summoned as a Noble Phantasm, can never reach even if it can temporarily glimpse it.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia