Legendary beasts
Not everything exists in the Empire closed-world campaign.
Gryphon
The gryphon is a beast that combines the aspect of an eagle and a lion. They are about the size of an adult ox with the forequarters of a great bird of prey and the hindquarters of a predatory cat. Despite their size they are not built to be ridden' there are tales of slightly built heroes and children riding on the back of gryphons but they are almost certainly fanciful. It is doubtful a gryphon could get off the ground with a rider, and even less doubtful that they would tolerate one for any period of time.
An adult gryphon is a match for an armoured knight. Their sharp talons tear through light armour to deliver crippling injuries. They favour hilly and mountainous areas that overlook fertile plains. A pride of gryphon will hunt across a large area, and they have been known to be a nuisance to animal herders in their territory. They mostly avoid humans but they are noted for their ferocity when threatened. They are particularly defensive of their young (which are born live, rather than in eggs) and there are stories of a pride tracking someone who has injured one of their cubs across hundreds of miles. They are capable of coordinated tactics, and work together to bring down larger prey; legends from Wintermark speak of the now-extinct gryphons of the mountains to the west of Skarsind hunting and bringing down adult mammoths.
These majestic beasts are very rare within the Empire; there are a few prides on the lower slopes of Urizen and the mountains of southern Kharaman but otherwise they have been hunted and driven out of areas claimed by Imperial citizens. Much larger specimens are believed to be found in the Realm of Summer, immense creatures that are powerful enough to be ridden by the Eternals of that realm; the gryphons of Summer are said to be intelligent, and capable of conversing. Their relationship, if any, to the beast found in the mortal realm is not known.
The gryphon is used extensively in the heraldry of Dawn. It is an enduring symbol of victory, prowess, pride and potency, and is strongly identified with the runes of Summer. Some Suaq hunters paint gryphons on their leather to draw on their powerful senses and hunting abilities. There is also some small trade in gryphon parts; magicians use feathers in rituals designed to grant strength, courage or pride and artisans are known to work talons, hide, bones and feathers into the construction of some enchanted items.
Griffon in Play
Gryphon provably exist. They are dangerous opponents, and their ability to fly makes them difficult to fight. It's worth reiterating that they don't serve as steeds; a gryphon would throw someone who tried to ride it onto the ground then tear them apart. They are rare in the Empire, but it is perfectly acceptable to roleplay that fur, feathers or talons are pieces from a gryphon, or to use pieces of them in ritual magic
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Unicorn