Unicorns.
One of the many things I've learnt from Red Dead Redemption is that a Unicorn can easily outrun an undead horde and can quickly build up enough momentum to plough right through them.
Also, Unicorns are usually characterised with above average intelligence and awareness, while Zombies have way below average intelligence, so I doubt a Unicorn would ever let itself become trapped by a zombie horde and fall prey to their weight of numbers... besides Unicorns are still large animals would never go into cramped environments (like buildings and corridors) where a Zombie horde poses the greatest threat.
Unlike that Horse on The Walking Dead, a Unicorn wouldn't allow itself to be ridden by an idiot into a horde of Zombies and left to their own devices, I think Unicorns could outwit and outmanoeuvre Zombies 9 times out of 10 (and probaby defeat them too, using hit & run tactics on lone Zombies or small groups).