The mysteries of common insults said by international players with broken English on the NA Server

(This thread is not intended to be racist in any way)

I just finished a game where all the stars aligned.

My top laner Riven (a very good Riven) was absolutely manhandling the enemy top laner, and by his first insult, I could tell due to his broken English and use of words that he was in fact a Chinese player.

He was playing Nasus and he said "r u dog?"

After we won a very one sided game (due to him tilting hard and doing nothing but trash talking [in Chinese 'pin yin' {e.g. rui wen etc etc etc ('rui wen' is Riven in Chinese)}]) even at the very end he still was completely oblivious to the fact that he just pulled the most ironic insult ever. It was so ironic it single-handedly put his own entire team on tilt.

As amazing as it was, I do have a genuine question, a question which even me, a semi-fluent Chinese speaker cannot understand.

After playing this game since Sept 2012, I really don't understand why the use of "dog" is such a common insult said specifically by Chinese players with broken English. I'm really curious as to why this is. Can anyone clarify this deep linguistic mystery?