Hello I am new to this game and was wondering if I can discard opponets attachments from play or just mine.

The effect of Mad King's Legacy doesn't specify the controller of the attachment that is to be discarded, so you can choose an opponent's attachment as well as your own. This is true for other cards, too: Compare the effect of The Power of Blood, which applies to all characters in play, to that of The Power of Arms, which only applies to your characters.

Also, note that the effect of Mad King's Legacy is a passive effect: You have to resolve it. If your opponents do not have any eligible attachments in play, you'll have to discard one of your own. If you don't have any either, nothing happens.

That is what I was thinking. Thank you very much for your time and wonderful answer.