You will get credit when the rogue is on mount. however it will be immune to the effect but if u check your achivement tab, it will still light up as completedI'm not so sure about this. I came across a dwarf and gnome rogue and they were both mounted. I was able to target them but when I tried to shoot it said "invalid targe" which suggeststo me that you can't shoot if they are mounted. It's very annoying as I am struggling to find these alliance rogues as it is!