Robert Burns is best-known today as an early Romantic or proto-Romantic lyricist, but he was also a collector of traditional lyric. He also loved traditional melodies, and much of his poetry is intended to be sung to airs far older than what he wrote; this is, of course, perfectly in keeping with the core notions of Romanticism, which are best spelled out in the preface to Lyrical Ballads (1789), which describes a poet's task as "fitting to metrical arrangement a selection of the real language of men in a state of vivid sensation" - something at odds with much of the poetry of the 18th century, which used deliberately archaic and learned forms. Burns did no such thing, preferring commonsense Scots just as Wordsworth favored everyday English.
A number of Burns' poems are light tweaks of preexisting ballads. "John Barleycorn" is likely one such, for example. In many cases, it's likely he wasn't even so much writing his own versions as attempting to present a version that would fit within an occasionally varied tradition.
The Merry Muses of Caledonia is a collection of such, printed ca. 1800 from a manuscript collection of verse Burns compiled throughout his adult life. This singular volume records the folk doggerel of the mid to late 18th century. The material in it likely does not originate with Burns, but he is known to have been fond of it and chose to compile quite a collection of such work, and it stands as an account of the 18th century's oral culture. The collection is united by a single common feature: every song in it is sexual in nature.
Bawdy verse is not the most prestigious area of folkloristic study, but there's no denying that dirty jokes are part of the present-day oral tradition, and similarly it is apparent that songs such as those in the collection were floating throughout the culture of Scottish inns and bars during Burns' lifetime. With the keen ear of one fascinated by the oral tradition, one of Scotland's great poets set these down in writing, and so they are available for us to study today.
Some of the songs in the Merry Muses have since, in less eroticised versions, continued to float in folk circles today. "Green Grow the Rashes" remains a well-known song, but I admit that I have never heard a modern singer perform a version which includes the word "cunt." I have heard "Sodger Laddie" done in a version much like that presented by Burns, but that piece of the collection is unlike some of the others in that the only sexual content is entirely implicit; the female narrator merely informs of what situation she was in when she "grew false" to her love.
Most of the songs, however, are far removed from anything that can be performed in what we now term "polite company," though they may cause great amusement in other settings even to this day if translated into more modern verbiage. Even some of the titles would be difficult to give in conversations with certain audiences in our somewhat more prudish day, such as "Nae Hair On't" and "Nine Inch Will Please a Lady."
Another oft-overlooked area of oral culture is parodic verse in which new words are written to an old tune for comic effect, and often directly reference the "original" version of the song to a greater or lesser degree. Much of this is found in the 18th-century barroom standards recorded in the Merry Muses, including "Sodger Laddie" (for which an earlier version which merely sings about the narrator's love for a soldier is still known), or "A' That an' A' That", whose refrain clearly references Burns' own popular "A Man's a Man for A' That" - a parody Burns clearly enjoyed, though it's unlikely he himself wrote verses about someone whose "prick stood like a rolling pin." The better examples of the style are remarkable for being humorous even to those unaware of the original, while being especially funny for those who do know the source material; "A' That" is exemplary in this regard.
Naturally, such verse continues to be written; I can only imagine what people 200 years in the future will think of ours.