Snark is certainly, not probably, a blend of snake and shark, because Lewis Carroll explained his coinage.
I've written about snark in the past, and I found this curious. The Century Dictionary gives the snake/shark derivation. It's repeated in other places too. But as far as I know, the only real evidence we have is the fact that Beatrice Hatch wrote that Carroll told her that it was a portmanteau of snail and shark. Where and when did Carroll explain that he coined the word from snake and shark?