Catoblepas
The Catoblepas is an obscure monster from Roman and Medieval European bestiaries, said to live in western Africa and generally assumed to be based on the wildebeest. Feeding on toxic plants, it resembles a long-necked, ugly, scaly-backed bovine, characterized most by the fact its hog-like head is too heavy for its long, slender neck, causing it to always look down at the ground - fortunately, as, depending on which version you read, either its gaze is immediately fatal or its breath is lethally foul-smelling.
The catoblepas is possibly the reason why the Dungeons & Dragons gorgon is a bull-monster that breathes petrifying gas, but the creature has appeared of its own accord in both Dungeons & Dragons and Pathfinder.
For D&D, the catoblepas has a surprisingly long history, and has appeared in every edition up until the fifth; first appearing in Basic D&D's The Strategic Review #7 (April 1976), and then in the Eldritch Wizardry supplement (1976), it went on to appear in the original Monster Manual (1977) for AD&D 1st edition and was upgraded with its own Ecology article in Dragon Magazine #73 (May 1983). It also appeared in the D&D Master Rules set (1985), and later in the Dungeons & Dragons Rules Cyclopedia (1991), in both cases renamed as the Nekrozon, for some reason.
When AD&D 1e gave way to AD&D 2e, it appeared in the Monstrous Compendium Volume One (1989), and was subsequently reprinted in the Monstrous Manual (1993).
Dragon Magazine again came to the rescue in 3rd edition; Dragon #299 (September 2002) and its "The Horrors of Cormyr" article was the first restoration of the catoblepas, which later became official in the Monster Manual II (2002).
Its final appearances thus far have been the D&D 4e Monster Manual 3 and the Pathfinder Bestiary 2, both released in 2010.
The D&D version of the creature has traditionally been based on the version described by Pliny the Elder and equipped with a lethal gaze, possibly to further distinguish it from the gorgon. Up until the 3rd ed MMII, its artwork has remained pretty much the same, in something of a parody of a sauropod; it has a hippo-like body and legs, a long serpentine tail tipped with a skull-crushing mace of bone, a serpentine neck, and a face like a hideously ugly warthog. The third edition changed the artwork for something... well, indescribable, but definitely not the same. They're all generally inoffensive unless threatened, but do tend to have a fondness for meat that comes up once a month. In the Forgotten Realms, in Cormyr, catoblepases are domesticated in one area, making the legendary "Death Cheese" from their milk.
The Pathfinder version, in contrast, is more like a huge, long-necked and really ugly pig with a toxic breath weapon. Whereas the D&D version is generally inoffensive, the Pathfinder version is a belligerent and malicious bullying thug of a creature.
The D&D 4th ed catoblepas is quite a different beast altogether. With an appearance more like a monstrous bull with an elongated neck and a hideously humanoid face, it is given lore as a native of the Shadowfell and a creature that enters the mortal world at the behest of the Raven Queen to serve as an emissary of tragedy, death and calamity. It comes in two versions; the Harbinger, which is bad enough with its poisonous breath, and the Tragedian, an even bigger and more deadly monster that has both poisonous breath and a lethal gaze. Both versions actually have some pretty awesome fluff, and certainly stand out compared to their counterparts.