Common name: Basilisk
Species: Serpens Regulus
More than twice the size of Titanoboa Exomantur, Serpens Regulus is the largest and most lethal snake I've encountered. With a skull that's more draconic than serpentine, it possesses a powerful bite, made even deadlier by the potent venom dripping from its fangs. The unique shape of its skull also allows serpens to hunt in a way that other snakes cannot: by burrowing. Once underground, it simply waits for its victims to approach. Fortunately its forked tail is usually still visible, poking just above ground.
Miraculously, survivors have not only managed to tame Serpens Regulus, but ride it. Some can even stay seated while it is lies in wait belowground, through I doubt it's a pleasant experience. While a wild Serpens can only be forced out of hiding by fire or explosives, once domesticated, it is easily trained to burrow or surface on command.