I just found/made a very nice macro for us hunters that have a problem with targeting mobs that our pets are fighting.
/target "pet name"
/assist
/cast attack
I made this macro because I was tired of shooting the wrong target, and having to cycle through the targets to find the one my cat is fighting. You can also use this in parties and raids. Just change /target "pet name" to /target "tank name" and you will never have to worry about shooting the wrong mob.