What is the main ingredient in the dish Welsh Rabbit?

The correct name of the dish is Welsh Rarebit, not Rabbit.

I have had it many times. A fancy cheese on toast. Easy on the mustard.

Ron, both names are correct with Welsh Rabbit being the earliest recorded in 1725. Welsh Rarebit isn't recorded until almost half a century later.

Ron, But rabbits make it a cute name.

Lol guys, they did wrote Welsh Rarebit. Look at the last paragraph. I think I'm blind but anyways that's fine to explain more than what they given...

According to some explanation in Wikipedia that I written in here... The first recorded reference to the dish was "Welsh rabbit" in 1725 in an English context, but the origin of the term is unknown. It was probably intended to be jocular. The dish may have been attributed to the Welsh because they were fond of roasted cheese: "I am a Welshman, I do love cause boby, good roasted cheese." (1542)[25] "Cause boby" is Welsh caws pobi 'baked cheese', but it is unclear whether this is related to Welsh rabbit. The word rarebit is a corruption of rabbit, "Welsh rabbit" being first recorded in 1725, and "rarebit" in 1781.[15] Rarebit is not used on its own, except in alluding to the dish.[15] In 1785, Francis Grose defined a "Welch rabbit" [sic] as "a Welch rare bit", without saying which came first.[26] Later writers were more explicit: for example, Schele de Vere in 1866 clearly considers "rabbit" to be a corruption of "rarebit".[27]