Why is 犬 used to refer to "dog" in Japanese?

Question

In Chinese there are two words that refer to "dog" - 狗 and 犬. In most use cases 狗 is more frequently used, as it refers to a specific dog.

In Japanese however, 犬 is used exclusively. Did 狗 exist in the past? If not why was 犬 picked over 狗?

Answer 1

From what I can gather, it seems like 犬 was the original Chinese word for "dog" and 狗 developed later, originally as a slang term for dogs in everyday life. (The radical on the left of the 狗 kanji is in fact a form of 犬.) Ultimately 狗 replaced 犬 as the general-use term, and the older 犬 is now reserved for more specialised usages such as established compounds.

It doesn't seem to be entirely clear when exactly this shift occurred, but I'd guess that during the period when the kanji were originally imported and assigned to Japanese words, 犬 was still the more standard word for "dog" in Chinese (or at least, 狗 had not supplanted it as completely as it has today), so 犬 was the natural choice of kanji to assign to the word いぬ. When the Chinese usage later diverged, there would be no pressing reason for the Japanese to change their established usage.

As for 狗, it doesn't seem to have ever been used as the primary kanji for いぬ in Japanese, though it is accepted as an alternative kanji for the word. I'd guess that being originally a more specialised kanji, it was naturally adopted for more specialised usages. I found some sources suggesting that 狗 can sometimes refer specifically to smaller dogs in Japanese, and its most common usage nowadays is in the name of the legendary 天狗{てんぐ} race.