There are also rumors that the grass one will be grass/dragon. Of course, by rumors I mean wild guesses. Also that rumor was likely started because of the Fighting/Psychic/Dark trio people have wanted since Gen II. That means it was probably just wishful thinking.Isn't there unconfirmed, yet plausible rumors about Tsutaja being Grass/Fight (Snake with arms makes it plausible), Pokabu being Fire/Dark (black part of head is like the whole "bandit" sort of thing), and Mijumaru being Water/Psychic(Eh, don't really see this one, though...)?
If anything, at this point these types are the best guess at the actual starter types, or at least their final evos.
At this point any secondary typing is possible. Some are more plausible than others based on the appearance of the Pokemon. I don't see how Pokabu could be part Dark. It doesn't look like a bandit at all. The fact that it is a bulky pig with a partially brown color scheme makes me believe it will be part Ground.
Those are far from the best guesses.