Why Orcs prefer Wyverns is a mystery lost in the depths of time, or at least a dark locker at GW. However, Wyverns have two wings and two legs, no front paws, so maybe that matters to Orcs.
A Flurry of Wyverns
FF54 Wyvern
FF61-1 Giant Wyvern
Fantasy Tribe Orc Wyverns
TA3 Orc War Wyvern
DRAG6: Orc War Wyvern
MB8 Orc War Wyvern
Orc Shaman on War Wyvern
Azhag the Slaughterer
Orc Boss on Wyvern (WM)
Orc Warboss on Wyvern
Orc Warboss on Wyvern (Unreleased)
Azhag the Slaughterer (v2)