There are a myriad of free-ranging, relatively fast-respawning rares all over Pandaria. All of them are required for one achievement, and part of a handful of others. All of them drop at least 1 piece of blue-quality gear, or a neat little blue trinket. Some drop epics, and some even drop mounts.
A few things to note:
- A rare is not "yours" until you tag it. Killing things around it =/= "your rare", as much as many people will consider that. Blizzard does not. Most people on this server are kind enough to not bother messing with it if they see people killing things around it, and I do my best to be one of those people.
- These rares have a ridiculous respawn rate. I've cleared my cache several times a day, and caught the same rare 4-5 times in one sitting. I've found them dead more often than not (because NPCScan alerts to dead mobs also). If you lose one to someone, it is not the end of the world. Given how many dailies are here, and how many times people will be flying through here, and the fact that the rare kills are AW, as well as factoring in the respawn time, you will get them again. Even Blizz will tell you that until you've tagged it, it isn't your rare. Spam whispering people/having people in your guild spam whisper them counts as harassment. You can get banned for that.
- It's really easy to tell which mobs have adds and which do not. Either by seeing someone else kill them, or looking around them. Chances are (the Mogu Warrior mobs, for instance, who have Stone Guardians around them) the adds are standing right beside them. Yellow neutral mobs are probably not going to pull with them, and as long as you don't AOE you're probably good to go. Feel free to clear them if you're concerned, but once again, refer to an above post. Until you tag it, it is not yours.
- There are tons, upon tons, upon tons of places that are phased based on quests completed. The occasion will pop up, I assure you, where someone will tag a rare and honestly not see you until the rest of the world catches up. Not everyone can afford a top of the line 2-3k computer that runs everything on ultra with no lag. Couple that with the extra server lag in some of the higher pop areas (Halfhill, Vale around Seven Stars, Jade Forest anywhere) and you're going to run into "hey, that -insert race/class/explicative here- stole my rare!".
There are hundreds of people on this server who are all trying to obtain rares - either for the achievements (like myself), for the mats out of the bags (for the crafters), or for gear (like many others I know). There is going to be competition, and while most people are nice, polite people who will stand there and wait for you to hit things, do not expect everyone to be this way. It's a fact of life that not everyone is a nice person, and it is also a fact of life that mistakes happen.
Don't be immature about it by blowing up people's whispers/having your guildmates do it for you when you get put on ignore. It's the easiest way to see yourself get reported. Don't be that guy.