This shape happens when you have two different thickness of batter - they don't need to be a huge difference, just very slight will do the trick. The white was poured first (a slightly thinner batter) and then the blue (a slightly thicker) was poured directly on top from a height. Because the white is more fluid, it will form around the "drop" of blue as it falls into the batter.Look at the photo of that soap closely. I think it looks like the white line is double in the tail section. Perhaps the blue base batter was poured along the sides of the white batter to forcing it up and in, which would make it surround the inside of the drop i.e. the way those giant drops in soap are formed. The similar drops on the side soaps lack a tail, but it looks like the white soap is wrapped around the interior of those drops. That could explain why the two top point don’t match up. Just an hypothesis.