Latin name: Dipsas
Other names: Dipsade, Situla
A snake so poisionous that its bite kills before it is felt

General Attributes
A snake so small it is not seen before it is stepped on, and so poisonous anyone it bites dies before he feels the bite.

Sources (chronological order)
Lucan [1st century CE] (Pharsalia, book 9, verse 867-895): “Tyrrhenian Aulus, bearer of a flag, / Trod on a Dipsas; quick with head reversed / The serpent struck; no mark betrayed the tooth: / The aspect of the wound nor threatened death, / Nor any evil; but the poison germ / In silence working as consuming fire / Absorbed the moisture of his inward frame, / Draining the natural juices that were spread / Around his vitals; in his arid jaws / Set flame upon his tongue: his wearied limbs / No sweat bedewed; dried up, the fount of tears / Fled from his eyelids. Tortured by the fire / Nor Cato’s sternness, nor of his sacred charge / The honour could withhold him; but he dared / To dash his standard down, and through the plains / Raging, to seek for water that might slake / The fatal venom thirsting at his heart. / Plunge him in Tanais, in Rhone and Po, / Pour on his burning tongue the flood of Nile, / Yet were the fire unquenched. So fell the fang / Of Dipsas in the torrid Libyan lands; / In other climes less fatal. Next he seeks / Amid the sands, all barren to the depths, / For moisture: then returning to the shoals / Laps them with greed — in vain — the briny draught / Scarce quenched the thirst it made. Nor knowing yet / The poison in his frame, he steels himself / To rip his swollen veins and drink the gore. / Cato bids lift the standard, lest his troops / May find in thirst a pardon for the deed.”

Isidore of Seville [7th century CE] (Etymologies, Book 12, 4:13): The dipsas is a kind of asp, called in Latin situla because anyone bitten by it dies of thirst. (Book 12, 4:32): The dipsas is such a rare snake that its trampled without being seen. It releases its poison before it is felt; it causes no grief to the one who will die because its appearance anticipates death.

Source: Medieval Bestiary.

all the animals of Satan’s bestiary, assembled in a consistory and set as guard and crown of the throne that faced them, singing its glory in their defeat, fauns, beings of double sex, brutes with six-fingered hands, sirens, hippocentaurs, gorgons, harpies, incubi, dragopods, minotaurs, lynxes, pards, chimeras, cynophales who darted fire from their nostrils, crocodiles, polycaudate, hairy serpents, salamanders, horned vipers, tortoises, snakes, two-headed creatures whose backs were armed with teeth, hyenas, otters, crows, hydrophora with sawtooth horns, frogs, gryphons, monkeys, dog-heads, leucrota, manticores, vultures, paranders, weasels, dragons, hoopoes, owls, basilisks, hypnales, presters, spectafici, scorpions, saurians, whales, scitales, amphisbenae, iaculi, dipsases, green lizards, pilot fish, octopi, morays, and sea turtles.

Source: The Name of the Rose by Umberto Eco.