Yes, indeed; a rare ability, and one supposedly connected with the Dark Arts, although as we know, there are Parselmouths among the great and the good too. In fact, his ability to speak to serpents did not make me nearly as uneasy as his obvious instincts for cruelty, secrecy, and domination.
-- Albus Dumbledore on the young Tom Riddle (HBP13)
A Parselmouth is a person who can speak Parseltongue, the language of snakes. It is a very rare ability (HBP13, CS11).
- Herpo the Foul (FB)
- Corvinus Gaunt (Pm)
- Gormlaith Gaunt (MNA)
- Marvolo Gaunt (HBP10)
- Morfin Gaunt (HBP10)
- Harry Potter (PS2), (CS11)
- Merope Gaunt (HBP10)
- Tom Marvolo Riddle (Lord Voldemort) (CS11, CS17, GF1)
- Salazar Slytherin (“Serpent-Tongue”) (CS11)
Parselmouths helped magizoologists discover the habits and personalities of the three-headed runespoor by conversing with them (FB).
Rowling stated that she didn't invent the term:
Q: Is Parselmouth a real thing or did you make that up?
A: Parselmouth is an old word for someone who has a problem with their mouth like a harelip. -
-- J.K. Rowling (RAH)
However, when I researched this I kept finding that the source for this was Rowling herself. I expect that if the term is an old word she re-used, it was either not spelled that way or is from some obscure dialect or slang.
From the Web
"Parselmouths" from Harry Potter Wiki