the sh- initial in 蛇 is a result of the th > tsh > sh sound change.
The same phenomenon can be seen in triplets that once were supposed to sound the same, such as 提 th / 㔭 tsh / 是 sh, 特 th / 持 tsh / 寺 sh, 濤 th / 鑄 tsh / 壽 sh, the list goes on.
There also the accompanying t > ts> s sound change such as 惰 t / 墮 ts / 隋 s.
I had a thread on this sound change if you want to see more examples.