Gen 1: Grass/Poison, Fire/Flying, Water.
Gen 2: Grass, Fire, Water.
Gen 3: Grass, Fire/Fighting, Water/Ground.
Gen 4: Grass/Ground, Fire/Fighting, Water/Steel.
While it's by no means a rule, it seems that any secondary typing in the past comes from the 'physical' types as opposed to the 'special' types. I wonder will this continue?
Also, it has been said, but
please stop using the argument that starters have to be effective against one another and that certain secondary types are disqualified because of this. The Grass/Fire/Water trifecta needs only occur in the starters' base form (if even) to show how types are effective against each other. I mean, even Bulbasaur was Grass/Poison in Gen 1.
My guess for Mijumaru would be Water/Fighting, as has been said. While the Fighting secondary type is over-used, it would be an interesting type combo when paired with Water.
For Tsutajaa, I'm hoping it won't be just Grass (I don't want ANY mono-type
) - maybe something like Grass/Dark (based on that imageboard sketch).
For Pokabu, it's difficult to guess where it's evolutions will go. If it does become a giant warthog, I'm guessing it'll be Fire/Ground or Fire/Steel. In fact, lots of potential type combos for Pokabu are afflicted by x4 weaknesses. It might surprise us and evolve into a Fire/Dark or Fire/Psychic. While I am tired of Fire/Fighting, I can see why it was chosen for the Fire starters in previous generations. Most secondary types would cripple a Fire starter.