In The Boys comics, the earlier Garth Ennis series that shares its fictional universe is Preacher.
The connection is revealed in the fourth volume of The Boys, where former vampire Proinsias Cassidy appears as a bartender. Cassidy is a major character from Preacher, the adult supernatural comic created by Ennis and artist Steve Dillon. His appearance turns what might otherwise seem like a hidden reference into explicit evidence that both series occupy the same continuity.
Preacher originally ran from 1995 to 2000 under DC Comics’ Vertigo imprint. It follows Jesse Custer, a Texas preacher bonded with Genesis, a powerful supernatural entity, as he travels across America with Tulip O’Hare and Cassidy. The series combines action, dark humor, religious satire, and supernatural fantasy.
A common mix-up is to identify another comics property associated with Ennis, such as The Punisher. Ennis wrote acclaimed Punisher stories, but The Punisher is not the earlier series named as sharing The Boys’ fictional universe. The television adaptations also have separate continuities, so the crossover applies specifically to the comics.