Shadowclaw is a level 13 aggressive rare cat that can be found just north of the Ruins of Auberdine in Darkshore. (I have indicated where he can be found in red on the map below.) He is especially desirable as a hunter pet as he looks exactly like the Striped Nightsaber mount as you can see in the screenshot above.
Shadowclaw can be found with NPCScan. You will need to add it, his NPCScan ID is 2175.
You can also use this macro to help find and tame this guy:
/cast Tame Beast
Although Shadowclaw is rare, he is on a very short spawn timer. It looks like his timer can be as short as 10 minutes since he was last tamed or killed.
Also, please be aware that he prowls around in stealth as you can see below.