Hi Miley! Those sites are interesting. I have not yet knit my dog a sweater, but I have a couple pattern books for them! That first site says if you can knit a scarf, you can knit a dog sweater, but that does not count that you have to know how to decrease. However, that is not hard! There are good videos on this site of various decrease sts.
You can do anything you want but you can't do everything!
FO: 2nd sock from defunct cable sock pattern saved from years ago... came out great, DD loves
OTH: Stash blanket Giant Granny Square in purples - getting bigger and bigger!