Juli Clover for MacRumors:

In the menu bar, there’s a simple Siri black and white icon that features the word “Siri” surrounded by a box, while the full dock icon is more colorful and features a colorful Siri waveform in the style of other built-in app icons.

I’m not into the dock icon for Siri, if that is what it ends up looking like. It doesn’t fit in alongside the other icons in the dock at all.

As John Gruber mentioned, why would Siri on Mac have an icon in the dock and one in the menu bar? It doesn’t make a ton of sense to me but, with the feature in the early stages, I’d assume final placement hasn’t been nailed down.