Rex Ashil is a level 56 rare wasp that can be found inside and outside of Hive' Ashi in northern Silithus. (I have indicated where he can be found in red on the map below.) He drops the typical green item and decent experience for levelers. He shares the same model as many other wasps throughout Azeroth, but he is the only one that is black in color making him a desirable hunter pet. He is tamable by hunters who are at least level 56 regardless of spec, and is a ferocity pet.
Rex Ashil is also one of the three rare spawns that can found in or around the Hives in Silithus.
Hive' Ashil - Rex Ashil
Hive' Zora - Zora
Hive' Regal - Lapress
Rex Ashil can be found with NPCScan. You will need to add it, his NPCScan ID is 14475.
You can also use this macro to help find and tame him:
/tar Rex Ashil
/cast Tame Beast
There isn't a lot of info on his spawn timer out there, but given that people don't really quest through Silithus any more there is a good chance that he'll be up if you were to go look for him.