I guess it's some type of trombone, but I can't find its correct name anywhere.
P.S. If you're wondering, here's where I got this shot from:
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This is a cimbasso (a valve contrabass trombone). The instrument is mainly used in Verdi's operas (and one opera each by Bellini and Puccini). The modern instrument has six rotary valves and is normally pitched in F, but there are also instruments in B-flat (a fifth lower). Both instruments can be heard here (the low B-flat is the one with the smaller bell, lower right):