The animosity between them is very odd, they're basically the same people. It's worse than the animosity between Yugoslavs because there's is based on religious conflict, but the Eritrean Habeshas and Ethio Tigrays have no meaningful differences.
Identities in the horn are fragile it makes perfect sense.
the British colonised northern Somalia, the people there got oppressed by Siad Barre and now a lot of them claim they’re Lander and not Somali.
Colonisation also helped create the different identities of Tigrinyas in Eritrea and Tigrayans in Ethiopia. Before the Italians came there wasn’t a definite border, regions in north Eastern Tigray and south eastern Eritrea were under one kingdom and even considered the same people.
Realistically speaking, Northern Tigrayans (Adwa, Axum, Adigrat) are more similar to Southern Eritreans than they are to southern Tigrayans (Timbien, Raya) . They speak a more similar dialect, look more similar and have a more shared history.
It’s all colonialism and petty beef. We are no more different than a Somali from Djibouti and Somalia. An afar from Ethiopia and Eritrea etc. but colonisation and past tensions make us identify as 2 different people.