The guitar is as good an instrument as any for folk music (sometimes), but following on from Mary's post we routinely go to gigs with 7 instruments* and none of them is a guitar. It's certainly not a necessity and I find it amusing that some people assume that in folk music an "instrument" has to mean a guitar, or even less comprehensibly, that a guy with a guitar is a typical traditional singer.

Anahata

* that includes two concertinas (different systems) and three melodeons. But I didn't include pipes, which I could justifiably add to the list on the basis of rare appearances.