You only get the cow plushie if you order the special collector's edition, which is only available through Natsume - that edition won't be appearing through any other retailers. Reserving the regular edition through Natsume will still only get you the yak.
Newegg has a special pre-order code for $10 off for any games that are $39.99 or more. code is : EMCNAJB243 It is good starting today for 3 days. If you choose to pre-order more than one game, then buy each game individually to apply the code to each one for the $10 off. Last time i used a $10 off preorder with Newegg on the Super Mario Bros. 2 it didn't include a pre-order bonus. But the $10 off made it worth it for me x2 since I pre-ordered 2 with the free shipping option. It got to me 4 days after release.
This is offering the yak. You can go to the Natsume message board and ask again, but it says so here. Someone asked, and a staff member replied: http://www.natsume.com/community/showthread.php?t=2428
It's really troublesome that they just won't put any details to the yak bonus right in the product description on amazon. It's not exactly a secret anymore, so it won't be a surprise. They're losing customers this way.
As a heads up to anyone, I contacted Amazon itself and it has said it has no knowledge of the Yak plush pre-order bonus. Whether this will permanently stay the case given Natsume's reassurance they will, I do not know. However, I am giving everyone a fair warning that at this point it may be best to consider your options if you desire the pre-order plush.
Always had problems with pre-orders for Harvest Moon, I never got the pre-order bonus of the Cow Plush when I pre-ordered Save the Homeland on early PS2! I am now 27 years old and I will NEVER LET THIS GO.