"And you think we're scum, do you? Scum who'll come running when the Ministry tells 'em to? See this [ring]? Centuries it's been in our family, that's how far back we go, and pure-blood all the way! Know how much I've been offered for this, with the Peverell coat of arms engraved on the stone? See this [locket]? Salazar Slytherin's! We're his last living descendants, what do you say to that, eh? So! Don't you go talking to us as if we're dirt on your shoes!"
-- Marvolo Gaunt (HBP10)
Marvolo was the patriarch of the Gaunt family, such as it is. He was belligerent and violent, with a paranoid streak. He shorter than his son Morfin, with odd proportions. He has “bright brown eyes, short scrubby hair, and wrinkled face.” Harry think he looks like a powerful, aged monkey. Marvolo wore a gold ring with the Peverell coat of arms carved into a black stone which he prized as a family heirloom, although he seemed to be unaware of the true significance of the thing (HBP10). Marvolo is the father of two children, Morfin and Merope. Marvolo Gaunt and his family were the last surviving members of the line of Salazar Slytherin. The centuries of inbreeding had reduced them to living in squalor in a filthy hut near the village of Little Hangleton.
Marvolo attacked Ministry personnel who had come to arrest his son for attacking a Muggle in 1925, so he too was arrested. He was convicted by the Wizengamot and sentenced to Azkaban for six months (HBP10). He died before his son’s sentence was complete.
"gaunt" = lean and haggard, especially because of suffering, hunger, or age.
Interestingly, Salazar Slytherin, from whose line Marvolo is descended, is also described as "monkey-like" (CS17).