I have only seen this word in The River War by Winston S. Churchill. An Australian website devoted to slang used by ANZAC troops at Gallipolli defines fantasses as ten-gallon water containers that are used in pairs and hung over mules or camels. This perfectly fits Churchill's context.
I have not found the word in any online dictionary or encyclopedia. I expect that it probably is to be found in the OED, but $295/year for an online subscription is a bit steep, even if it is the world's best dictionary.