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
Azhag the Slaughterer (v2)