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 - and rather fortunately, as, depending on which version you read, either its gaze is immediately fatal or its stank 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 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), which was... rather odd to say the least. In the article it laid out that the catoblepas as we know and love is simply the female of the species, and that the male is a much more lithe and sprightly creature reminiscent of a kangaroo, but is just as vulnerable to the female's toxic nature as any other creature. The only way the species seems to propagate is that the female, when in heat, begins to exude pheromones that attract the males, making them put aside their normal drive to not die. They will wait until the female's head is buried in the swamp muck feeding and then bum rush to tap that ass before she can look up and kill them.
The catoblepas 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 the horrid beast appeared once more in the Monstrous Compendium Volume One (1989), and was subsequently reprinted in the Monstrous Manual (1993). They completely tossed out the odd sexual dimorphism from Dragon Magazine #73, going back to its more standard ecology as lumbering bog grazer, but giving the creatures an occasional hankering for carrion or flesh that strikes during full moons.
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 became official in the Monster Manual II in the same month. It remains pretty in-line to the 1st and 2nd edition renditions, but is said to have been created by some asshole wizard kit-bashing critters together in the same fashion as the bulette. Also, the range of its death ray vision is nearly tripled!
It went on to appear in the D&D 4e Monster Manual 3 and the Pathfinder Bestiary 2, both released in 2010, and ultimately appeared in Volo's Guide to Monsters for 5th edition.
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 Edition 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. In the Forgotten Realms, in Cormyr, catoblepases are domesticated in one area, making the legendary "Death Cheese" and "Deadeye Butter" 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 Edition 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.
In 5th Edition, the catoblepas resumed its classic appearance, even being outright referred to as a combination of "bloated buffalo, dinosaur, warthog and hippopotamus". This version is a Challenge Level 5 Monstrosity, which sports a cudgeling tail, a stench so powerful it can inflict the poisoned condition, and a death ray gaze attack.