Behirs are a strange monster native to the multiverse of Dungeons & Dragons. They can most simply be described as a gigantic cross between a blue snake and a centipede that spits lightning. No, seriously; they're giant blue-scaled snake-things with multiple pairs (at least six) of reptilian legs - they can either run along on these many limbs or fold them flat against their sides and slither along like enormous serpents. Their head has often been portrayed as crocodilian, usually with dragon-like backward sweeping horns, but they have the ability to dislocate their jaws like a snake, allowing them to swallow smaller creatures whole and alive. They are sapient and even capable of speech, but are fairly brutish and simple-minded.
Behirs commonly inhabit warm, hilly regions, often living in caves high in hills/cliffs(or other shelters above ground level), with a territory extending over 400 square miles. Fortunately, they're a rare creature and unlikely to be encountered - which may have something to do with their fanatical racial enmity for dragons. No, seriously; they hate dragons and try to kill them wherever they encounter them. Especially blue dragons.
Ironically, it has at least been suggested that behirs might have been derived from draconic stock in some ways. Which would explain their tendency to collect hoards.
Whilst none of their variants have gained any major traction, three behir subspecies are known to exist. The jungle and desert behirs are simple environmental variants of their hill-dwelling cousins, but the Halruaa region of the Forgotten Realmss is known to be home to the simply named Halruaan Behir, which is smaller and weaker than a normal behir, but also smarter and with an innate affinity for magic.
The behir debuted in Advanced Dungeons & Dragons 1st edition in the adventure module The Lost Caverns of Tsojcanth (1982); one year later it was officially added to the Monster Manual II. It was first updated to AD&D 2e in the Monstrous Compendium Volume One (1989) and from there made it into the Monstrous Manual (1993). An article in Dragon Magazine #156 (April 1990) added the Jungle and Desert Behirs.
Dungeons & Dragons 3rd Edition saw the behir debut in the Monster Manual for both 3.0 and 3.5, with Dragon Magazine #333 featuring the Ecology of the Behir. The Halruaan Behir debuted on WotC's website, but was then printed in the 3e Shining South splatbook in 2004. In Red Hand of Doom the half-fiend Behir Varanthian serves Tiamat and her Hand and will begrudgingly work with the Hand's other dragons if they all survive till the rematch.
As an OGL monster, Behir appear in Pathfinder. They're slightly beefier than their 3.5 incarnation, but otherwise pretty typical. The penultimate crunch book, Heroes of Golarion, included the Beast Speaker feat that lets a character take a behir as an animal companion, but like most ways of getting a non-animal as an animal companion in PF, it's garbage and worse than just having a big cat, which doesn't require a feat, as a companion.