Doctor Leonard McCoy came from a family which had been abducted by aliens called 'skagarans'.
Here is a quote from Wikipedia:
"Throughout the franchise, McCoy is depicted as a physician of considerable skill but preferring old-fashioned, personalized or less-intrusive remedies to cutting-edge treatments and computerized medicine; in "The Deadly Years" (1967), he describes himself as "just as an old country doctor". He also believes in the body's own recuperative powers, though he also realizes that medicine as it exists in the 23rd century has to help along the process.
McCoy is suspicious of technology, especially the transporter, and occasionally is bigoted toward Spock's Vulcan heritage."
Here is a quote from the memory-alpha website:
"A group of men on horse take another man and hang him. They refer to that man as a Skag. They shoot their guns in the air and ride off.
While searching for the Xindi in the Delphic Expanse in 2153, the NX class starship Enterprise, commanded by Captain Jonathan Archer, discovers a planet inhabited by Humans in a setting that resembles the late-nineteenth-century American West. Wearing period dress, members of the Enterprise crew visit the planet surface and learn that the Humans are descended from Americans abducted from Earth in approximately the 1860s by a species called the Skagarans who, after bringing them to this planet, used them as slave labor. T'Pol, Trip and Archer scout out the town to find useful information. Reed, back on Enterprise, informs the captain that there is a smaller town filled with aliens only ten kilometers away. Archer tells T'Pol and Trip to go to the town and find out what they can, while he heads off to a local bar. Trip and T'Pol go off to find the town, but Trip thinks getting a horse first will speed up the journey. Trip finds a horse dealer and trades him his gun (a gun from the nineteenth century period) and a harmonica."
I suggest that McCoy's grandparents were there and were later re-located to Earth where they permitted to live in a deliberately retro styled community. This is, of course, speculation and non-canon.