You might want to break it down into categories of information:
Take said cow and disassemble it into its attributes:
-Mammal: As a subset of the Animal, Humanoid, Monstrous Humanoid and Giant types, but with no game role. It declares that they have hair, warm bloodedness, give birth live, and produce milk, none of which is relevant in a mechanical sense.
Any mammal would find these traits to be extremely common knowledge, as they possess them personally, and can verify them with personal examination.
-Gross physical traits: Size, horns, hooves, color. These are not necessarily common knowledge, but easily acquired through observation. Just as you need no knowledge check to tell you that the dragon in front of you is fucking huge, you need no knowledge to know that the creature in front of you is big, horned, quadrupedal and hoofed.
This also means that knowing about these attributes is automatic on sight, common knowledge to those who have seen it before(though they may know it by another name), and a harder roll for those who've never seen it before.
-Type: This goes under one of the information points of the 10+CR check. A Type and all it implies are part of one special ability/vulnerability. Yes, you might not be able to positively identify a cow as the Animal type. You can call it an animal anyway, but you do not know about animal HD size, skill points, BAB and save progression if you fail the check.
-Detailed physical traits: Yes its big and strong. How big, and how strong is a specific creature ability check. Whether this particular cow has a trample attack is a specific creature ability check. All of these have the CR based formula in play. Remember that the CR based knowledge DC goes up rapidly from each additional ability/vulnerability you want to know about, if used exactly in the manner Cyclone Joker specifies, it opens an undesirable loophole in that when I roll a special ability knowledge check on a red dragon, the DM gives me the following information: It lays eggs and its scales can be used to make armor, oh look, you ran out of your two items of information.
Finally, you need to consider checking against a named creature(just a name), an observed creature(seen, described or heard), an encountered creature(engaged in extended interaction such as combat), and a rumored creature. Each of these are very different checks with very different difficulties, reflecting very different levels of available information. The more intimate your interaction, the more info you have to work with.