I'll keep it in mind then, not to say "Seu cavalo" when i come to Portugal.
I know this was a while ago, but I think the reason "Seu" can mean "you" in some dialects of Portuguese is because in that case it comes from "Senhor" so sometimes it can be a more formal or polite form of address. But according to wiktionary, the normal seu (derived from Latin suus meaning his) can also be used before insults for emphasis. So in the case of something like "Seu idiota!" it would be like saying "you idiot!" We do something similar in Spanish with "so" but I don't know if this is just parallel development or if they share an etymological root.