Etrigan, often simply called "the Demon", was created by the legendary Jack Kirby and first appeared in THE DEMON #1 (1972). Supposedly, Kirby based his image on a demon which appeared in the PRINCE VALIANT newspaper strip.
The Demon's origin would have been told in the "Swamp Thing Meets Jesus" story by Rick Veitch. In the never-published story, Etrigan was born of the demon Belial, but he was exorcized by Jesus Christ from the human he was gestating in. As a result, Etrigan is somehow bound to Earth and often acts as it's defender. Belial's other son, the wizard Merlin, bound Etrigan to the immortal human Jason Blood to keep him from the clutches of Morgan LeFay (see issue #87.)
Etrigan often spoke in rhyme since his first appearance. Alan Moore reasserted and refined this idea in SAGA OF THE SWAMP THING #25, having the Demon speak in iambic pentameter. Since then, writers (the good ones anyway) have tried to maintain that rhyming pattern.
Notable appearances in SWAMP THING:
SAGA OF THE SWAMP THING #25 "The Sleep of Reason..."
SAGA OF THE SWAMP THING #26 "...A Time of Running..."
SAGA OF THE SWAMP THING #27 "...By Demons Driven!"
Jason Blood, the Demon, shows up at the child care facility where Abby works to deal with the terrifying Monkey King, the Kamara.
SWAMP THING ANNUAL #2
Etrigan helps ST retrieve Abby's soul from Hell.
SWAMP THING #50
Etrigan dons his armor to battle the Ultimate Evil, alongside a dozen other of DC'c mystical heroes.
SWAMP THING #76
Etrigan prevents the Phantom Stranger from interfering during the conception of ST and Abby's child.
SWAMP THING #85
In the form of the immortal human Jason Blood, the Demon takes an interest in the Claw of Elk-Hound and ST as he travels back through various eras.
SWAMP THING #87
Consistant with Etrigan's origin story in THE DEMON #1 (1972), King Arthur's wizard Merlin uses his Eternity Book to bind Etrigan to the mortal form of Jason Blood in order to protect the Demon from falling into Morgaine LeFey's power.
In a story that was never published, Etrigan's birth is shown in a plot involving the arrest of Jesus.