I was certain there had to be a Horde mount-style scorpid out there. It turns out there is, but the colour I found isn't the same khaki brown colour as the mount (UPDATE: people have found him in ALL of the other colours as well now... see the end of this post). The one I found has the dark blue skin, with light blue spots, which looks pretty cool. And the colour is stable. Despite the wild NPC spawning in different possible colours, he doesn't colour-change when tamed. This is good that he doesn't, but it may take hunters a LONG time to find the exact colour they want.
His name is Madexx and he's a level 84 non-elite, enormous scorpid, who wanders the Sahket Wastes in northern Uldum. I'll provide a map shot at the end of the post.
Here he is: And a screenshot of the tame: He sprays a pool of poison, which you're kind of stuck standing in while you complete the tame (unless you freeze him first), but it didn't seem to hurt me too much.
Here's a screenshot of him as pet sized: And here's a map shot of where I found him: UPDATE: Irie found exactly the same mob, same name, in the same place, but had the brown skin. See posts on page 2 of this thread.
UPDATE 2: Artemisia can confirm the green skin as well. See page 4.
UPDATE 3: Holy59 found Madexx in black. See page 6. Just the red left to find!
UPDATE 3: aaaand that completes the set. Artemisia found Madexx in red! See page 7.
If you find Madexx in a different spawn location, could you please report in with a screenshot of your map and mention the colour you found?
