This non-subtle but punny postcard from a century ago plays off the etymology of nightmare, which, when used in the sense of a bad dream, only dates to the 1820s. The mares, though, weren't literally horses. They were considered to be demons or goblins that "rode" on people's chests while they slept, thus causing frightful dreams (or potentially death, in some cultures).
The word "mare" has a confusing etymology that involves Old English, Norse, Germanic, French and perhaps all the way back to Greek. It was thought that mares could haunt (or ride) more than just people. They could leave horses sweating and exhausted or tangle the branches of a tree. In Slavic countries, some of the methods of repelling nightmares included leaving a broom upside down behind the door or placing a belt atop the bedsheets.
This postcard was mailed in 1918, during the 20-month period when postage was raised from 1¢ to 2¢. It appears to have been mailed to Al Guffey in the unincorporated community of Nettleton, Missouri. The message, clearly written by a child, states:
The weather is fine here. It is New Year's day. I got a watch [and] a pair of gloves for Xmas.