Aha! Unique as in separate creatures, rather than unique as in one of a kind. Gotcha.
Of all the snakes we encounter, mambas are certainly unique in how we deal with them. All other venomous snakes get caught, put in a box, and delivered to the regional snake park. That only happens for baby mambas. Adults get left well alone

.
That's right, it's not "Unique" in the AD&D "Frequency" monster stat sense.
The AD&D Black Mamba in
Dragon #237 can move faster than a riding horse in open ground (30 vs 24) faster than a heavy warhorse in other circumstances (18 vs 15) and bites so rapidly it gets two attacks per round. The AD&D version in
Dragon #115 has three bites per round, but at least it's a bit slower at move 15!
The Dragon #237 Black Mamba also has special rules for its venom.
Oh, incidentally, while going through your index to match the snakes I noticed a few missing entries:
- Aeserpent - we've done a CC conversion for this since your last Index update.
- Mystaran Cobras - the Index has Vaults of Pandius conversions of the Black Cobra and White Cobra but not their BECMI originals. The Black Cobra is on page 58 of Champions of Mystara: Heroes of the Princess Ark [Explorer's Manual]. The White Cobra's on page 33 of HWA3 Nightstorm (which also has a standard Cobra and Baby Cobras on p 32).
- The 3E Minor Flame Snake and Lesser Flame Snake are both missing their "Fiend Folio + 3.5 update" entries.
- The Ice Viper just has its 3E Bestiary of the Realms 2 conversion, the original is in Ruins of Undermountain [Undermountain Adventures] page 25.
- The Quarrel Snake also originally appeared in Ruins of Undermountain on page 117 of the Campaign Guide.
- The Pharaoh's Ruling Snake originally appeared in Pharaoh (I3) on page 32. It's also in the Desert of Desolation reprint but I'd have to do a bit of digging to find the page number of that.
Both the "Ruling Snake" and "Quarrel Snake" are actually magic items, like the Staff Snake on page 1295 of
Encyclopedia Magica Volume IV. That's also missing its original appearance (the Encyclopedia lists its source as the Rules Cyclopedia, but it also features in other BECMI rulebooks' magic item descriptions).
If you're listing the monster form of the
snake staff, shouldn't you be listing the monster form of the
python staff form of the
staff of the serpent? That's basically the same creature and is in multiple D&D books, including multiple editions of the
Dungeon Master's Guide.