I have been caught up with this lately. I feel like it is something I must have known and then forgotten. I get asked a lot of questions about why some things are the way they are in English. Usually I will just answer, "Because it is English." If the questioner is somewhat of a sophisticate, I will expound on how English is a bastard child of various European fluctuations. I didn't realize, or had forgotten, that a lot of our synonyms, or near synonyms are a direct byproduct of 1066.
The explanation is probably slightly too simplistic, but goes something like this: After the Francophone Norman Invasion, French became the language of the ruling classes in England. Most of the commoners, however, didn't speak the language, conversely, the ruling classes didn't speak so much Middle English. They also didn't do so much raising of livestock, only consuming it. Consequently, words describing the livestock remained through to Modern English in their Middle English forms. Words for the meat product of those animals reflect their French ancestors. Hence: Cow/Beef.
Here is a further explanation.
Here are some examples:
pig/pork, cow/beef, wood/forest, sheep/mutton, house/mansion, worthy/honorable, bold/courageous, chicken/poultry.