hi Michi,
you can see some mande horns here (you can even buy one if you are fast)
http://cgi.ebay.com/Old-Tribal-BAMBARA- ... 0176579606see the first pix (included bc the page will soon disappear)
http://www.hwmconline.com/Instrument.php?inst_id=249see the second pix
the second web includes a section of african aerophones which might be interesting to look at.
There is some spelling variation re. the name:
butuwo (-u) - horn, trumpet (from a malinke dictionary)
Further variation can be seen in a rhythm listed in djansa.be
Burubila-foli / Burun
Burubilafoli, meaning "trumpet ensemble music" is another rhythm/genre, played with djembe, chun, 7 trumpets, hand clapping and wooden clappers. The buru (horn trumpets) are historically tied to the kingdom of Segu, where the king called for the trumpets to be sounded at every victorious battle, but also when deaths occured in the battlefield. Nowadays it is played at the death of a village chief, to welcome a notable person and during the Ramadan and Tabaski festivals. Burun; the horn. Apart from old Dunbia, nobody in Bamako still recalls this Rhythm, which was used to accompany the play of trumpets
btw, this is a wild speculation but the pronunciation bears some resemblance to kudu horns (jewish antilope horns)
The rhythm namani might also be helpful when researching on horns in wassolon. also hunter's trumpets are used in sangbarala in dunun(ba) rhythms. I would think that originally a horn was used instead

cheers
[edited to add the pix]