My horse has bumps/rash on his dock, what could it be?

Many things, from dirt rubs to sweet itch. Get it checked by a vet! It's dangerous asking medical advice over the internet. You don't know who you're talking to, or what their qualifications are. You might be getting good and correct advice, or you could be getting told something that would be fatal. Even if it is a vet you're talking to, there's only so much they can do from a computer screen.
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