Behirs are long, lightning-spewing snakelike creatures that eat people alive and hate dragons. Their intelligence is subhuman, but they're still smarter than mindless predators and can even speak common.
Obviously, a behir has got good melee potential. RHD that grant full BAB and a ten-sided hit die, Huge size, great strength and constitution, reach, 40 ft. movement, natural armor, and a bunch of grappling abilities.
Also of note is the breath weapon. It deals some (7d6) damage and can be enhanced with metabreath feats, making it a reasonable use of your action if you can't reach your foe for whatever reason.
The final question remains: what LA is appropriate here? I'm inclined to +2 LA myself. Not only is +2 a decent rule of thumb IMO, it also allows behirs to get 7th-level maneuvers should they enter a ToB class.
Next time: the monster you've all been waiting for! The sphere of many eyes! The eye tyrant! The annihilator of adventurers! That's right: the beholder!