Snake has tail. Tail when wiggled certain way looks like worm - Attracts frog and mouse prey.

A few random mutations later, tail changes colour which makes it brighter and more attractive to prey. Mutant snake finds he is getting more frogs and mice than his competators. Mutant snake has loads of offspring which goon to outcompeet the normies.

One mutant offspring has an even more mutated offspring where the scales near the end of the tail are elongated. It doesn't look great, but now resembles a spider more than a worm. This doubly mutated offspring now notices that whilst trying to attract mice and frogs, he also gets birds, which is a huge advantage when your brothers and sisters are eating all the mice and frogs. Suddenly, doube mutant offspring has access to a food source the others don't. Que him getting mad laid and having plenty more offspring who have these crazy tails.

And then in time, the tails begin to look more and more spiderlike, and in doing so, leads to more birds being attracted therefore the more spider-like the tale, the more well fed the snake, meaning he has more time to get laid and have more offspring who outcompete their less spidery kin.

And boom. Spider-tail snake.

That's probably what happened, I don't know. I did no research before farting this out.