Fictional musical instrument from the fantasy short story "Bethmoora" by Lord Dunsany, published in the collection A Dreamer's Tales, 1910. A percussion instrument, apparently. It is not described.
It also appears in the Dunsany's short story "Idle Days on the Yann," from the same collection:
Then I entered Perdondaris and found all the people dancing, clad in brilliant silks, and playing on the tambang as they danced.
The first quote is discussed by Lord Dunsany in his 1938 autobiography, Patches of Sunlight:
In that tale comes a line that escaped from the obscurity that seemed in those days to wrap the rest of my work, and was sometimes quoted. I used of course to invent names for things in use in my unknown lands … On this occasion I threw down three invented names in a heap, rather perhaps in the spirit in which Beethoven amused himself with the calls of the quail and the cuckoo in the 6th Symphony; they were the names of musical instruments, and the sentence went, 'In little gardens at the desert's edge men beat the tambang and the tittibuk, and blew melodiously the zootibar.' As I wrote at the same time as what was known as the Irish renaissance, and as I am Irish, some vaguely associated me with it, and the tambang and the tittibuk were even thought to be Irish instruments.