I have never seen “Tamang” (i.e. Tamang people in Nepal) written in Tibetan. (a) Today, I learnt that dGe-’dun-chos-’phel has spelled it as rta mang (“many horses”). It is a true bahuvrīhi (’bru mang) compound. (b) Out of curiosity, I looked up Wikipedia, and it provides the Tibetan word rta dmag (“horse army”). (c) But there is yet another interpretation (i.e. of Ajitman Tamang), which is badly represented there but obviously he meant mtha’ mang/dmangs (“border people/subjects”). It would also be possible to spell it as tha mang or tha dmangs (as in tha gru).