The tips you trimmed off were the softest part :<0| Think frayed and split ends making them thinner more easily bent. Most of the horse knots I shopped for were mixes of mane and tail. Never bought one. Like you I always wanted to make one and know where the horses are. Just never got around to it. To get the shape the hairs are stacked upside down into a tighter shape of what you want the finished product to be, bulb, fan, whatever.

You did good. You made your own knot and even if it's scritchy at first you may break it in to a nice soft feeling brush. I have some vintage with horse and if to long they are floppy as an old wet mop. Let us know how it performs !