Step 15: Use the curved lines on the right as a guide to draw the rows of spikes on top of the alligator's tail. Follow the basic path of the line as you draw the small, wide triangle-like shapes across the tail. When you get to the middle, where the tail curves, cut across it and continue drawing the spikes on the top line. Keep drawing the spikes until you get to the tip of the tail where they become smaller. Add a second row of spikes that starts at the base, then curves around and meets the first row of spikes at the front.
Step 16: Finish up the tail by simply darkening the initial guide lines.
Step 17 (optional): For a cleaner look, erase as much as you can of the initial guide lines. Don't worry about erasing all of the guides. It's okay to leave some behind. Re-draw any final sketch lines that you may have accidentally erased.
Final Step (optional): You can add more value throughout your alligator drawing for more detail. To get a rough texture, use random scribbles instead of smooth shading. The top part of an alligator tends to be darker, while the underside is usually lighter. The bottom part is also less bumpy, so don't add the squiggles there. Instead, draw grid-like lines for the scales. Don't overthink it when adding the rough texture on the skin. It seems more difficult than it is. Just add random scribbles everywhere. Add the less bumpy, gridlike scales on the belly too. Vary the pressure on your pencil to get different degrees of tonal value. Keep the light source in mind when adding the value so that the shadows are consistent. Adding the value can be quite time-consuming, so be patient and take breaks. It's always a good idea to use reference for a more accurate drawing.
Add a cast shadow underneath. This helps ground the alligator so it doesn't appear to be floating. Use a darker value near the middle of the shadow and a lighter value along the edge for the shadow's diffusion. Don't forget to pause the "How to Draw an Alligator" video after each step to draw at your own pace.