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The word Rong is transferable to Yanyun (猃狁) which means Rong was more likely of Xiongnu (later name) origin.
The idea that the Xianyun 猃狁 were later called the Xiongnu appeared relatively late in Shiji commentarial traditions, as a misinterpretation of Sima Qian's (probably erroneous) claim that before the Xiongnu arose on the steppe, there were Xianyun and Mountain Rong living there. Actually, what little we know of the Xianyun from Western Zhou sources suggests they lived in the Gansu Corridor and not in the northern steppe. The Xiongnu themselves never claimed any relation to the ancient Xianyun.
A recent book by Li Feng of Columbia University, about Western Zhou history, argues that the Xianyun came to be known as Quanrong 犬戎 in Eastern Zhou times, based on the canine connotations of 猃. That said, there is no evidence that the Quanrong were ethnically or linguistically akin to other tribes of Rong in the Shaanxi-Gansu region, or to the 'Mountain Rong' (or 'Northern Rong') of northern Hebei and Shanxi. As I said, the Rong label creates an illusion of homogeneity where there was actually much diversity.
BTW, I don't believe that the Di 狄 peoples were closely related to the Xiongnu either, but we have had that disagreement on another thread before.
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The later Qin was already a country famous of horse breeding which was a typical Xiongnu trait.
Isn't it too simplistic to assume that horse-breeding was a
uniquely Xiongnu trait? The Shang and Zhou probably bred horses for their chariots, and some have even suggested that horse domestication spread to China via Central Asia and Gansu long before there was any nomadism on the northern steppe. Besides, the association of Qin with horse-breeding arises from the Shiji's account of the very origins of the Qin state, when the first Qin lord was supposedly enfeoffed by the Western Zhou king Xiao (Ji Pifang, c. 891-886 BC) and given the surname Ying as a reward for his skill in breeding horses for the Zhou court.
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Jiang (姜) and Qiang (羌) wrote same in oracle bone script, which means at least in Shang era they were not distinguishable.
My oracle bone script dictionary (by Wang Benxing, Beijing 2006) shows that they were written slightly differently. 姜 was a kneeling human figure with ram's horns, while 羌 was a standing human figure with ram's horns, sometimes holding a stalk of grain. In fact, the oracle script for 姜 was quite similar to the right side of the script for Ji (the surname of the Zhou kings), except that the kneeling figure in Ji had a sort of topknot between the ram's horns.
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Though the ruling class of Zhou might be of early Sinicised Huaxia (华夏) origin
I would question whether using such terms as Sinicized and Huaxia makes much sense in the Zhou context, when there was still no fixed 'Sinic' culture and no concept of 'Huaxia' as an ethnic identity. In fact, throughout the Zhou and Qin-Han periods there is not one piece of evidence from the sources that shows that the people of the Zhongguo region referred to themselves as Huaxia. 'Xia' yes, 'Hua' yes (in the Zuozhuan/Zuoshi Chunqiu), 'Zhuxia' yes, but never 'Huaxia'.
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The surname Ying (嬴) is of Dong-Yi (东夷) origin
That it was used by eastern Yi does not mean it could not have been adopted by members of other groups. Indeed the Shiji gives a coherent explanation for why the Qin lords had the Ying surname: the Zhou king supposedly gave the surname to the first Qin lord as an allusion to one of the sage-king Shun's ministers, who had received the surname Ying as a reward for raising many livestock.
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One example is Sanxingdui (三星堆) culture, another is the Yi (彝) history which has mixed origin both from Qiang and Dong-Yi.
I have never seen evidence that the Yi of Yunnan had anything to do with the Qiang or eastern Yi. Could you show me some?
As for Sanxingdui, I have only seen arguments that Liangzhu-type items found there indicate trade with communities in the Lower Yangzi. No one argued that this alone could be evidence for eastern Yi migration to Sichuan.