Lay a square over a circle and use the use the Add To Shape Area function to make a single shape. Arrange the teeth around the mouth, copying-&-pasting as you go.
You can change the shape and size of each tooth as you go round, as well as rotating them to fit the curve of the mouth. Copy-&-paste the top row of teeth, and flip them 180-degrees for the bottom row.
Select the mouth shape and teeth, and copy-&-paste it. Click the Divide tool in the Pathfinder palette; this splits everything into separate pieces. Delete everything from the mouth apart from the teeth. Delete the teeth from the original mouth and replace with the cut-out ones.
To make the mouth more detailed add different shades of ellipses to create depth in the mouth. To make a lighter shade, reduce the amount of black in the colour, and for a darker shade, increase it.
Draw some drip shapes using the pen tool in a lighter colour to create monster saliva. Once you’re happy with the mouth, drag it onto the monster shape, and position it.
Next, add some more detail to the monster. Draw veins by making curvy lines with the Pen tool. Use a lighter colour and various stroke weights. Try to make the lines as smooth as you can by manipulating the anchor points and pulling the handles on each point.
This will take practice, but once you get a feel for how the handles affect the curve of the lines, it will become easy. Remember to turn Smart Guides off when drawing with the pen tool.