One species I haven't seen mentioned at all is the Grivet (
Chlorocebus aethiops), which, although not exclusively restricted to the Ethiopian highland, doesn't occur much outside of it:
Chlorocebus aethiops (Grivet Monkey)
And, although I don't know if they occur in the highlands or not, the Beisa oryx and Grevy's zebra also occur in the area:
Oryx beisa (Beisa Oryx)
Equus grevyi (Grevy's Zebra)
I can't find them in Ethiopia in iNaturalist, but the Neumann's grass rat does occur there according to the IUCN, and could make for a nice smaller mammal addition:
Arvicanthis neumanni (Neumann’s Arvicanthis) (They're not held as much as other species, but Chester zoo has a very large group of them according to ZTL)