Cow - ng'ombe, Bull - ng'ombe, Ox - ng'ombe, Cattle - ng'ombe, Cows, Bulls, Oxen - all still ng'ombe. Beef - Nyama ya Ng'ombe. Ng'ombe jike - now that really is just a cow.
From what I could find: ng'ombe - cattle in general ng'ombe dume - bull (lit. means cattle + male) ng'ombe jike - cow (lit. means cattle + female) ng'ombe maksai - ox, bullock (lit. means cattle + castrated animal)
This consonant ng' is one sound consonant, just a different sound of "n" (nasal velar, /ŋ/) as in English "sing", and the second sound is a prenasalized "b" with two sounds (m+be). The stress is on the first syllable ng'o.
Here some pronunciations:
https://www.youtube.com/watch?v=BntsggNJt28 (Learn Swahili Try 1/2)
https://www.youtube.com/watch?v=t9rIE8jdJuQ (Learn Swahili Try 2/2)
From Wikipedia: https://en.wikipedia.org/wiki/Velar_nasal