Of course, Moombas are explained within the context of the game. In the Shumi Village you can learn who the Moombas are, who they evolve from, and so on and so forth. The game takes the plot developments of the dream very seriously and treats them all as truth, which makes the dream theory especially difficult to argue. Sure, all this stuff seems weird, but how do we know that it’s all intended to be a dream? Maybe the creators just thought it’d be cool to have talking cats around! And who knows, maybe they did. But I think it’s more than convenient that the more fantastical elements, such as talking lions, do not appear in the game until after the moment where Squall may have died.