Step-by-step: Bring out the beast!
1 Gesture drawing is the foundation on which the whole painting depends, so do it as many times as you need until that scribble becomes a scary monster. Move the shoulders around, lower the head… experiment aplenty. These should be fast and loose so you can crank ’em out.
2 You can use lighting to accentuate the silhouette, which should relay the notion that this thing isn’t human. Here I’ve given him a triangular head, amphibious fingers and the lower body of a snake to hammer it home, Then I back-light both characters so he reads fairly easily.
3 Exaggerate the threat. One of the reasons I’ve given Slippy here such a large jaw is so he could fit that girl’s entire head in his mouth. That’s not to say he will (she may have super powers, we don’t know until he goes for it), but exaggerating the threat creates more tension.