The Energon Pub

His original toy was one of the figures imported from the Japanese pre-Transformers Diaclone line - it was version of the same truck used for Optimus Prime, but with a different trailer. Unlike the original figure, where the trailer opened up into a base / communications centre, the figure used for Ultra Magnus combined with its trailer into a larger, armoured robot.

Since then, as a 'homage' (read: overused cliche in just about every toy line going, especially if it's aimed at collectors), there's usually a white repaint to follow shortly after any Optimus Prime release, with the name Ultra Magnus.

As to why the character looks like Optimus Prime... that's never been explained, although the Dreamwave comics series suggested that he was Optimus' brother-of-sorts. In the original G1 version and the recent IDW comics reboot of its continuity, Magnus is never shown without his armour - it effectively transforms as part of him, so he doesn't really have a 'white Optimus' mode and the confusion doesn't arise.