The fictional book The Windflower by Laura London, set around 1813, has the following claim:
"If there's a pig on a ship, everybody calls it Dennis," Cat said. "Don't ask me why."
There's a forum thread on a Civil War website that makes a similar claim.
There's also this 1907 article from the Calexico Chronicle referencing a ship pig named Dennis, as well as the following quote from Kevin J Hayes's 1999 book Melville's Folk Roots:
Pronouncing the word "pig" aboard ship was sure to bring forth bad luck; instead, the sailors referred to the pig as "Mr. Dennis," "Little Fellow," or "Gruff."
Was Dennis the pig a thing? Are all these just a coincidence? If Dennis the pig is a thing, when did that start and why?
Thank you!
What a fascinating question. I can't explain why a pig, specifically, would be considered bad luck, but Kevin J. Hayes in Melville's Folk Rots (1999; I found the same source as you) suggests that looking at pigs as bad luck was common among sailors. This is what we would call occupational folklore. Hayes describes fisherman who won’t sail if they encountered a pig on the way of his boat. This is like some miners’ folklore who won’t report to work if they confront a cat (sometimes a cat of a specific color) on the way to work. The same goes for a redheaded woman! Understanding why various things inspire these sorts of folkloric concerns is difficult, and speculation often results in its own brand of folklore – people did that because (fill in the blank). While these e3xplanations are put forward as certain, they are almost always 99 percent speculation and 1 percent something other than certain!
It appears that the use of the name “Dennis” for the pig was what folklorists and anthropologists would call a circumlocution. These are common when people deal with something they regarded had some power to express ill-will if not referred to with courtesy. The assumption here is that using the proper name of the thing would be offensive.
My Swedish mentor, Sven S. Liljeblad (1899-2000; see my brief article Nazis, Trolls and the Grateful Dead) was collecting place names from a Shoshone informant near Pocatello, Idaho in 1942 when he asked him the name of a mountain. The man said he would not name the mountain within sight of the place but would instead use a more polite term to refer to the mountain. Sven persisted, and his informant relented. They immediately had a flat tire, walked into a nearby small town for help, and were arrested for being German and Japanese spies. Sven said he would respect the wishes of informants after that!
Thus, bears were referred to as “sweet foot,” “honey eater,” brown one,” or “dear uncle.” The fairies were referred to as the “good neighbors,” “the gentry,” “the wee folk,” or the “hidden folk.” Many animals and things were perceived as warranting these circumlocutions. Referring to the onboard pig as Dennis appears to be part of this approach – which is international (but not necessarily universal – that’s another matter).
As to why Dennis or why pigs? That’s a great question. I don’t have the answer, and I doubt anyone does.