Silverwing

Silverwings are a subspecies of Griffons, that live in the Silver Mountains. They are not as bulky or strong as normal Griffons, instead, they are highly agile and very quick making them effective mounts for hit and run tactics, scouts and being messengers. They nest in caves where they can hide their eggs from any would be predators. They live off of whatever birds they can snatch from the sky and hunting for smaller mammals like deer and on occasion hares.
The Larrentell family starts the training process the minute the chicks hatch. Someone who has proven their worth must help the mother take care of the Silverwing chick day and night, after they have taken the Silver Oath . When it reaches 2 years of age it is able to fly and the real training starts here, needing to build it's stregnth, stamina and loyalty. Having grown up with their trainer and rider Silverwings only allow one person to ride them freely. However, as was proven at First Landing you can tame an adult to ride on if necessary.
They are outfitted with Blacksteel armour, filled with weight reducing Runestones since they can't carry both armour and rider. The riders wield 4 meter long lances made of black steel, slotted with Runestones reducing weight and knockback, greatly lowering the chance of dropping the lance in a fly by. They are a deadly force, flying all over the battlefield in a flash cutting down enemies and disappearing into the clouds before another attack.

Statblock: Silverwing
Young statblock: Young Silverwing
Chick statblock: Silverwing chick

(Art Credit: https://www.reddit.com/r/worldbuilding/comments/i8izxv/griffin_creature_concept_sketch/)