A Slughorn is a fictional Medieval instrument invented by Thomas Chatterton (1752–1770), a precocious and tragic lad who forged Medieval poetry in the guise of fictional monk Thomas Rowley, and committed suicide at 17.
The word seems to be Chatterton's misinterpretation of the archaic Scottish Gaelic term slughorn, or sluagh-ghairm, meaning "war-cry", which gives us our word "slogan." Taking it for a real horn, he used it in several poems.
In one about the Battle of Hastings:
Throwote the campe a wild confusionne spredde;
Eche bracd hys armlace siker ne desygne,
The crested helmet nodded on the hedde;
Some caught a slughorne, and an onsett wounde;
Kynge Harolde hearde the charge, and wondred at the sounde.
"some caught a slughorne and an onsett wounde" meant "some picked up a slughorn and sounded a charge". A slughorn in this context appears to be some kind of trumpet. However, in a footnote to another usage of the word, Chatterton explains it is "a musical instrument, not unlike a hautboy." But a hautboy (or hautbois) is an oboe and not a horn. However the idea of sounding your war-oboe before a battle is awesomely hilarious.
The last stanza of Robert Browning's 1855 poem "Childe Roland to the Dark Tower Came" has, in a deliberate nod to Chatterton, a slughorn:
I saw them and I knew them all. And yet
Dauntless the slug-horn to my lips I set,
And blew. Child Roland to the Dark Tower came.
- Thomas Chatterton, The Rowley Poems, Hastings ii.90 and footnote 15 to Eclogue the Second, at Project Gutenberg.
- Robert Browning, Browning's Shorter Poems, selected and edited by Franklin T. Baker, A.M., Macmillan, 1917 at Project Gutenberg.