The Horn was never referred to as Zanj. It's a popular misconception but not true.
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(Bantus/Nilotics who may have been mixed with Southern Cushites includes the Masaai who live in previous bilad al Zanj)
Description of the Zanj from al Muqqadasi and Ibn Battuta.
From Ibn Battuta's visit to Kilwa (A Zanj colony)
Funnily enough Ibn Battuta also visited Mogadishu at the same time. He didn't describe it as a Zanj or Swahili city.
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Even the earliest description of Somalis stated us as a separate group on different from the Zanj.
View attachment 12109 Where the Arab got their slaves from "the Zanj, a Bantu speaking people's from East Africa" again not Somalis. When Ibn Battuta visited Somali Sultan Abu Bakr he noted that Abu Bakr spoke both Arabic and Mogadishan, which is Somali.
Funny how Abyssinians, Sudanese, Nubians, Bantus and other Blacks beyond the Sahara were mentioned as the source of slaves for the Arabs but not Somalis