A
Tanzanian language of uncertain
affinity, possibly
Cushitic, but it may be
Nilotic. It has about 32 000 speakers. Why it is particularly
noteworthy is that it was long believed to be
Bantu like all its neighbours, because among other things it used the Bantu system of
class prefixes. The Mbugu language is also known as
Ma'a.
It appears however that it borrowed this whole system comparatively recently -- a striking refutation to anyone tempted to say that fundamental grammatical structures do not get borrowed. However, it is perhaps the only example known of a language that has been completely reprogrammed in this way: it reminds me of an adult cell that's been turned back into a stem cell then given a new direction.
One clue is that the prefixes are not used consistently in all circumstances, whereas in a true Bantu language they would be.
The Mbugu also speak the Bantu language of their neighbours the Pare, and keep their own language, sometimes called Inner Mbugu, for ceremonial purposes. Maarten Mous of the University of Leiden has made the extraordinary suggestion* that Inner Mbugu originated as a language game, or a kind of secret language, invented for just such ceremonial purposes. Such things are known -- ceremonial sub-languages in Australian Aboriginal languages for example, with different lexicons, but these are unmistakably highly restricted in their use. No other example is known of an invented style of language, if that's what it is, that has become anything like a normal language. But, pidgins become creoles spontaneously, so I suppose it's not an absolutely impossibility.
* Mous, M. The Making of a Mixed Language: The case of Ma'a/Mbugu. 2004. John Benjamins. Reported in New Scientist, 7 February 2004.
See also http://lings.ln.man.ac.uk/Info/staff/WAC/Papers/Mixed.pdf for more about mixed languages, and criticism of Mous's hypothesis.