Shawls can be rectangular too, like overgrown scarves, and they're usually called stoles. They can be semi-circular or a full circle.
To make a triangular shawl from the tip to the neck, you'd start with about 3-5 sts, inc 2 every other row. When you got to 10 +1 + sts for the edges (sort of k2, yo, k10 +1, yo k2) you can start the pattern. The next RS row, you'd only have 2 more sts, so you'd mark off the beginning of the horseshoe pattern and let the new stitches add on to either side of it, and just repeat the pattern over the 10 +1. When you get another 20 +1 sts on, (10 on each side) then you can do 3 repeats of the pattern across the row, and so on. It might be better if you look up a simple triangular shawl pattern first and understand how they're contstructed before you attempt to design one of your own. Lotsa patterns at 336 Triangle Shawl Patterns
You can also look at the top down triangular patterns at elann.com. Several very pretty ones there.