A Hippogriff (also spelled Hippogryph and Hippogryphe) is a legendary creature, supposedly the offspring of a griffin and a mare. Ludovico Ariosto's poem, Orlando furioso (1516) contains an early description (canto IV):
XVIII

No empty fiction wrought by magic lore,
But natural was the steed the wizard pressed;
For him a filly to griffin bore;
Hight hippogryph. In wings and beak and crest,
Formed like his sire, as in the feet before;
But like the mare, his dam, in all the rest.
Such on Riphaean hills, though rarely found,
Are bred, beyond the frozen ocean's bound.

XIX

Drawn by enchantment from his distant lair,
The wizard thought but how to tame the foal;
And, in a month, instructed him to bear
Saddle and bit, and gallop to the goal;
And execute on earth or in mid air,
All shifts of manege, course and caracole;
He with such labour wrought. This only real,
Where all the rest was hollow and ideal.

According to Thomas Bulfinch's Legends of Charlemagne
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Like a griffin, it has the head of an eagle, claws armed with talons, and wings covered with feathers, the rest of its body being that of a horse. This strange animal is called a Hippogriff.

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The reason for its great rarity is that griffins regard horses as prey. It has been suggested this idea was strong enough in medieval times to produce an expression, "to mate griffins with horses", which meant about the same as the modern expression, "When pigs fly". The hippogriff was therefore a symbol of impossibility and love. This was supposedly inspired by Virgil's Ecologues: ... mate Gryphons with mares, and in the coming age shy deer and hounds together come to drink.., which would also be the source for the reputed medieval expression, if indeed it was one.
Among the animal combat themes in Scythian gold adornments may be found griffins attacking horses.
The hippogriff seemed easier to tame than a griffin. In the few medieval legends when this fantastic creature makes an appearance, it is usually the pet of either a knight or a sorcerer. It makes an excellent steed, being able to fly as fast as lightning. The hippogriff is said to be an omnivore, eating either plants or meat. Another description of the Hippogriff can be found in Arnold Sundgaard's poem, The Hippogriff:
When Mare and Griffin meet and mate
Their offspring share a curious fate.
One half is Horse with hooves and tail,
The rest is Eagle, claws and nail.
As a Horse it likes to graze
In summer meadows doused in haze,
Yet as an Eagle it can fly
Above the clouds where dreams drift by.
With such a Beast I am enthralled,
The Hippogriff this beast is called.