My alternate e-mail address in Hotmail has been used to create a Yahoo ID which I don't know...?

[Casca Longinus]

First, I hope you are smart enough to never give yahoo/hotmail, or anyone online that is not for anything IMPORTANT any real info, always lie.
Second, go into your mail accounts, and change your passwords. Do not use the link provided in that email, log into the official site. Make sure there are no alternate emails that you are unaware of, if there are, delete them, change your security questions/answers, and passwords.

**************************

OK, you are probably right about the phishing scam, and that's all it was. I would think that someone might want your username, but it was already in use by you at yahoo, so they would of coarse try that username at a different email, but that does not explain why you would get an email informing you of the new email with the same username as your yahoo. I believe, as you do, that it was just a phishing attempt, ignore it.