In minecraft i saw a zombie in golden armour?

Along with Skeletons, some spawned Zombies can pick up dropped items from the ground. These Zombies will automatically hold any item they come across, but if those items happen to be Armor, Weapons, or Tools, the Zombie will wear them. Some of these Zombies spawn already sporting such items, and those items may also be enchanted.

Sometimes zombies/skeletons can spawn in armor (Gold or leather). They can drop their armor sometimes, but other then that they are the same as a zombie without armor (Other then the protection the armor gives the zombie).

It's a rare spawn.