Did Jagex forget that Vorkath only has 3 legs?

Different Vorkath, different number of legs.

Mod Dragon spoke about it during the livestream we had back when Vorkath was first introduced - if memory serves, we were hoping to create an entirely bespoke set of animations (walk, stand, lay, run, fly, attack, die (again), you name it) with 3 legs, but with the time we had we elected to use the existing rig we had from Astellarn as a base, and made (new!) animations with that but that meant that we had to keep Vorkath with 4 legs.

Fourkath? Is that anything?

Anyway, we stuck with the 4 legged rig and Mod Dragon didn't have to amputate.