ptolemy18 said:
I agree, "Fair warning" is the best way. But that comes down to DMing style -- I doubt there can be some rule in the Monster Manual saying "If you plan to use the Catoblepas, it must be surrounded by "WARNING: CATOBLEPAS" signs within a 500 foot radius."
Looking at the description in the old MM, those warning signs would be something like "Roughly 500 feet ahead of you, you see a small herd of very weird animals. Massive bodies, long tails equipped with some sort of knob on the end, very long and thin necks that curve downwards to a head that is mostly under the surface of the swampy water they are grazing in. Bob, your wizard seems to have heard of such, roll for Intelligence to see if he recalls those beasts. John, same goes for your swamp ranger, but with a +2 bonus."

Coupled with the basically low chance of the catoblepas to actually raise its head enough to bring the death effect into play (25% when the group is still, 10% if the group moves fast), this made the beast into a curious, odd aberration that raised more story questions than combat challenges.
Looking at the new MM2, the description is still there, but there is nothing that keeps the DM from having the catoblepas gaze around at each character. Although it says that it tends to keep its head close to the ground, it uses its Death Ray in self defense. So basically that's another beastie that has become more deadly with the new edition.
It's funny, the more I compare the older editions with the latest one, the more I see why a lot of problems that pop up in 3.X simply weren't there in that extent in older editions. Not meant as an edition war remark here, mind you, just comparing. I wonder how much of the dislikes today would have been avoided if there had been some different decisions 10 years ago regarding the design of 3E.