I was cantering my new ottb yesterday and for the first time he really bent/bowed his head down low in the canter. But how low is too low? I've heard of horses going underneath the bit, but does anyone have any advice on the proper head/neck position? It felt like he was pulling too much but it was hard to tell. I would like to show him eventually. I would love any advice or video on proper form.
Their poll should never be go below the saddle. But the headset has nothing to do with the horse being on/behind/in front of the bit. You want the their head to be perpendicular to the ground and them to be pushing forward from their hindquarters. Someone will explain it better than that I'm sure.
Also, be aware that while riding your horse you can't see them from the side, so if the horse always holds its head high when he drops it the head may feel lower than it actually is.
I don't know how much your horse has done but if you are just starting him off the track its more important getting him relaxed and stretched out than having the "correct" head position. On the other hand having his head too low may make it harder for you to give rein aids, and he may just be avoiding you, it will also negatively effect his balance.
With my past horses when they were doing similar things or avoiding things I found circles helped. I find circles actually help almost anything. But keeping him light on the rein and guiding him around turns and figure eights should help him to keep his head up as he will have to keep his balance more. For me horses shouldn't really be pulling at all, there should be a contact, but the horse should be holding itself up not leaning on your reins.
My horse started doing that to me too. She would pull her head down at the canter and go faster, basically running away with me a bit. She would yank down and then I would lose the leverage that I needed to be able to sit up and pull to halt her. I know a lot of people, like another poster said, don't like aids, but I feel that sometimes they can be necessary (even for just a little bit), to help the rider correct a problem. So, I tried her out in a corkscrew bit and have not had any problems since. In fact, she is softer and lighter and we have never meshed better. I have gone back to the d-ring snaffle and she does fine and if she starts to get heavy again I switch the bit back. If your horse is not respecting your cues to stop or slow down, you need to change something and gain that respect back. Good luck.
I have had her a year and am so lucky with how willing she is. I use a 3 piece snaffle, the more you pull on a race horse that faster they go so I try and use a softer bit so that if I do pull on her a little, I am not giving her the wrong idea.