Before Ford Prefect was born, his father lived on Betelgeuse Seven - until the Great Collapsing Hrung Disaster. He was the only survivor on the planet. After that, he moved to Betelgeuse Five, where he fathered Ford. The kids on B5 nicknamed him Ix, which means 'Boy who cannot satisfactorily explain what a Hrung is, nor why it should choose to collapse on B7‘. But what is the Hrung? Is it mentioned in one of the other books in the series?
It wasn't just Ford who was unable to explain what a Hrung is; nobody really knew.
The only place the word Hrung appears in the entire series is in the first footnote in Chapter 5 of the novel The Hitchhiker's Guide to the Galaxy, part of which says (emphasis mine):
Ford's father was the only man on the entire planet to survive the Great Collapsing Hrung Disaster, by an extraordinary coincidence that he was never able satisfactorily to explain. The whole episode is shrouded in deep mystery: in fact no one ever knew what a Hrung was, nor why it had chosen to collapse on Betelgeuse Seven particularly.