WoWDB

Portent is a rare spawn Quilen with that spawns in one of four random colours - blue, red, green, or purple.

He is tameable (but as an exotic - only by Beast Mastery hunters), but patrols in stealth.  The way to find him is to search for his tracks throughout the Vale, follow them and drop a flare when you see a new one appear.  Put a Hunter's Mark on him as soon as you see him appear, because he will disappear again as soon as he leaves your flare. 

The tracks are really easy to see on the yellow ground and will have a "Dusty Tracks" tooltip on them when you mouse over.  Before 5.1, the tracks faced backwards and not actually the direction the mob was walking, but this has since been corrected.

He will not attack you while you are taming him but he can walk out of range, so you might want to try to get in front of him before starting.

Quote from daetur »

He can, if memory serves, be trapped to buy more time to tame him before he paths out of range.

Yes, that is true also.  These mobs do walk fairly slow, so as long as you don't start taming from max range, you should be okay with or without the trap.

If you throw the mark up so you don't lose them to stealth again, you should have plenty of time to throw a trap or position yourself.  The only real time sensitive thing is getting that Hunter's Mark up as soon as you can.