Slughorn can refer to several things and one (fictional) person.
It is an obsolete form of the word slogan, closer to its derivation from the Scots Gaelicsluagh-ghairm (meaning war-cry).
In turn this influenced the pseudo-Medieval poetry of Thomas Chatterton. For example in a poem about the Battle of Hastings he writes "some caught a slughorne and an onsett wounde" (Battle of Hastings ii.99), meaning "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 defines it as "not unlike a hautboy". The Medieval English word hautboy is the origin of the modern word oboe and has never referred to any instrument comparable to a trumpet.