I need some advice. I bought a horse a couple of months ago, an ex racehorse, gelded late and a bit mouthy, but nothing major.
In the last week this has escalated to quite aggressive behaviour, he greets me in the field when I go to catch him with his ears pinned back, he tries to bite me when I put the headcollar on. He also pins his ears back when I approach him when he is tied.
Before I start my little rant, please know I am not saying this to discourage you, but you need to see this connection here.
The parts I bolded ARE connected. A "bit mouthy" is not "nothing major." Horses do not think a "bit mouthy" means nothing. He was TESTING you. He got away with the small stuff, and now it has "escalated to quite aggressive behavior."
Aggression is an escalating thing. RARELY does a horse wake up one day and say "hmm I'm going to be aggressive today." He has been testing you since day one, as all horses do, and you have proven to him that HE is boss by letting him get away with being "mouthy."
What you need to do is learn the difference between respect and disrespect at ALL levels, whether it be slight or outward. You recognize that pinning ears is bad, GOOD! Go after his little butt and let him know who is really boss.
Have you ever watched 2 horses work out their dominance issues? Your horse sees this situation as no different than that. He thinks he's your boss, and so far he is correct. What you need to do is recognize his subtle, or not so subtle signs, and nip them in the bud while you can. It will get worse otherwise.
Signs of disrespect: Pinning ears, kicking out, squeeling, rearing, turning hind end, not wanting to be caught, not respecting personal space, dancing on the lead rope when leading, rushing when leading, stopping to eat whenever they feel like it, not moving out of your way when you ask for it, being MOUTHY (including nipping, biting, messing with clothes, or having his head in your space), food aggression, tail swishing, and I'm sure I missed some. Some are subtle, some not so much. The point is, each one needs to be treated as a NONACCEPTABLE behavior.
They kick each others butts to establish dominance, by lashing out. Do NOT let him do that to you, make that little sucker run his butt off next time he tries it. Go ahead and get angry with him, run him around that pasture until he's dripping sweat. He'll get the message. He will NOT be easy on you, so don't be easy on him. Does that mean go beat him? NO! BUT if he needs a good whack or two, that won't kill him, trust me. Get his feet moving and his brain thinking and you should see a vast improvement in just a few days.