Not sure if this is a known issue or if it's caused by the dawnguard add-on but whenever I equip an ebony shield it appears invisible and also cannot be stacked on shield racks but interestingly can be displayed on mannequins is there a fix for this?

Alas it is a bug, not just a glitch. Like some other items of armour/clothing the ebony shield will be invisible for all vampire characters due to those races being forgotten to be added to the ebony shield's list of playable races. As it is there isn't/won't be a fix for it on the 360 or PS3 unless they address it in a patch. PC players can fix it themselves via the Creation Kit.

Lol how do they forget to add that? I hope its patched although I doubt it will anyways I dont really use ebony shield's just trying to make some cool looking shield racks.

My guess is that the game was rushed out the door before more comprehensive quality assurance could be done. It's also why bugs like the small levelled list error that produces the occasional naked courier (Skyrim day one bug) and the Breezehome alchemy lab can't be purchased after Dawnguard is installed were allowed to slip through.

What I don't understand is why these very quick and easy things to fix are never fixed in the patches. It's almost as if they think the problem doesn't really exist if they don't acknowledge that it does. Never understood that and I probably never will. At least the PC players can fix those things themselves, but it still leaves the console players out in the cold.