Natsume (the Harvest Moon creator) really seems to enjoy their preorder bonuses and their plushies. Nearly very game they’ve released within the series (and recent memory) has had some sort of bonus available.
Currently, this seller has the Cow (Save the Homeland bonus), the Dog (Another Wonderful Life Bonus), the Sheep (Magical Melody bonus) and Finny the Fish (no idea where this one is from). Another seller has the Black Cat plush which came from Harvest Moon DS. I have a feeling too, that there are probably more of these than I realize.
There are also two games coming out for the series soon, Harvest Moon: Island of Happiness (DS) and Harvest Moon: Tree of Tranquility (Wii). Looks like they will have their own bonuses as well. The Chicken that will be coming with the DS game (Amazon preorder only apparently) looks pretty awesome, but then again every single one so far is damn cute.