The snake is most appropriately the mascot of Slytherin. The beauty of the snake lies in its twining shape, just as the beauty of the Slytherin path lies in intricacy, in unexpected twists and turns, mirroring the curves of the letter S.
The path of the Slytherin may well be winding, curving back on itself so that an onlooker cannot tell where the Slytherin is bound, in what direction he is going. The Slytherin is not restricted to straight lines, in order to achieve what he desires. For this reason, he has options which are not available to others. This makes him a valuable weapon.
We must also consider the tongue of the snake – famously it is forked, read by the enemies of Slytherin as a sign of deceit – the double tongue, the ability to say one thing and mean another, the capacity of deceit. And certainly, these are Slytherin gifts. But like all gifts, this may be applied for any purpose – are there not times when trickery is absolutely necessary, when one’s motives must be known only to oneself?
Because of the double tongue, a Slytherin may remain unpredictable, and operate in darkness – and only when the game is finished can anyone safely say on which side he has been playing.
I found this in the fic The Double Tongue by rowen_r and I've added it to my userinfo page, because it's just so right for me.