Putridus the Ancient is an elite rare mob found in Icecrown. Due to his gigantic girth, he can be seen even from a good 10,000 yards or so, making him an easy mob to spot from such a distance. His only known moves are War Stomp and Putrid Punt
Putridus the Ancient is a criteria of the achievements:
Putridus is one of the 2 mobs that can be seen from a good 10,000 yards or more. The other is the considerably larger Thrym.
One reported spawn location is at Mord'Rethar:The Death Gate