What's a "Filet of fenny snake," you ask? It's a side cut of any snake that lives in a fen, a swamp in England. This snake deserves to stay in the fen, not in your occult cauldron, you heartless witches!
What the hoo-ha is an "Adder's fork?" Well, an adder is a snake, and a fork means a forked tongue. So it's a snake's tongue. To make this brew, these witches must have yanked one out of a slithering snake's snatches and tossed it in the pot! They were tough, there can be no doubt.
A blind worm? Oh, Shakespeare means a slow worm, which is a limbless reptile often referred to as a blind worm. These things can regrow their tails when chopped off, but that's still no excuse to do it!
The witches add the "maw and gulf of the ravin'd salt-sea shark," but didn't they know how endangered sharks are! These witches were not ecologically conscious! Come on, witches, reduce your footprint!