Catoblepas

This revolting creature has the swollen body of a huge buffalo on short, thick legs like a hippo's. A long, serpentine tail whips with startling speed and power. On the end of a long, feeble neck sits a head like a warthog's, only far uglier, with enormous bloodshot eyes.

Its tail strikes for 1-6 with a 75% base chance of stunning the target for 1-10 rounds, reduced by 5% for each level above first, or each Hit Die beyond the first for monsters.

The gaze is far worse: a death ray reaching 60 yards. Any creature that meets its eyes dies with no saving throw. If the creature surprises a party, a 1 in 6 chance exists that someone has already locked eyes with it. Shutting or averting the eyes does not grant immunity, but it allows a save vs. death magic.

Because the neck is so weak, after the first round the catoblepas has just a 25% chance each round to lift its head and use the ray. That chance grows by 15% per round while it and its target both hold fairly still, and falls to 10% if it must track fast movement. Several together will work to drive their prey into overlapping lines of sight.

Most of the time it plods around its marsh grazing on reeds and grass. Roughly once a month, usually at the full moon, it craves meat, and that is when adventurers tend to run into it. Its lair is a sheltered patch of firm ground, typically ringed by tall reeds, and it seldom fears intrusion since it is usually the most dangerous predator in the swamp.

Catoblepas mate for life, so a group is always a mated pair, with a 10% chance of one youngster that has half an adult's Hit Dice. The young take nearly nine years to reach early maturity, and females give birth only once every 10 to 12 years; both parents rear the calf.

When hunting it eats fish, eels, marsh birds, water rats, snakes, big amphibians, and other swamp life, typically stunning prey with its tail and killing it with its eyes, and it will eat carrion too. Being semi-intelligent and living 150 to 200 years, it learns from past encounters. It treats humans warily, sizing them up, and normally attacks only when hunting or protecting its mate or young. The coins, gems, and occasional magic items in its lair are just leftovers from beaten intruders; it cares nothing for them.

Its deadly gaze is protection enough that nothing preys on it.