Catoblepas (plural):
I am a big fan of the basic Catoblepas found in Volo's Guide to Monsters, as well as the (slightly) revised version in Mordenkainen Presents: Monsters of the Multiverse. It is delightfully weird, but I could never find a reason to include it in an encounter. It just never really fit.
And now that I finally found a place for it, my players are too high level for these weird swamp death cows to give them any sort of challenge. Unless I had them fight a horde of 12 of them, or something. But that's just... A bit boring.
So, I decided to upgrade the classic catoblepas, turning it into two separate variations: the Stinkeye and the Foulmouth. Where the Stinkeye is basically just a souped up base catoblepas, the Foulmouth instead deviates from the original. With a disease laden bite replacing the tail attack of the original, and some AOE -capability coming from the breath weapon serving as its recharge attack, the Foulmouth is a slightly more versatile (and threatening) swamp cow, while staying true to the (stinky) theme and flavour of the original.
Both of them pack a bit more punch than the original, with flexible multiattacks that take into account the recharge abilities of each version. It's just always more fun when you don't have to choose between a multiattack and the cool *special* attack. With these catoblepas, you can do both!
Sight Rot
The Sight Rot carried by the foulmouth's bite attack is not detailed here, as it is a part of the 5e Basic Rules, and can be easily found on the internet for free.