These are in online dictionaries and described as ‘imitative’ and ‘child’s term’ but how far back do they go in English? Who first got in print with one and what was the context? And are there any others? (There is ‘baa-baa black sheep’ but only in the nursery rhyme I think.) Do other languages have equivalents?
The most famous use is maybe Joyce’s opening sentence:
‘Once upon a time and a very good time it was there was a moocow coming down along the road and this moocow that was coming down along the road met a nicens little boy named baby tuckoo ’
Researching this I also came across an idiom ‘Why kick a moo cow?’ which was new to me and wasn’t explained though I take the meaning to be ‘easy target’ stuff.