A “Salty Dog” is a cocktail of gin (or vodka) and grapefruit juice, served in a glass with a salted rim. It is not known who invented the drink or where, but a 1947 citation of “Salty Dog” comes from Texas—and Texas is a grapefruit-growing state. The salted-rim glass is also used in a Margarita cocktail.
 
A “Perro Salado” (Tequila Salty Dog) contains tequila instead of gin or vodka.
   
   
Wikipedia: Salty Dog
Salty Dog
A Salty Dog is a cocktail containing vodka or gin and grapefruit juice, served in a glass with a salted rim. The main difference between the Salty Dog and the Greyhound is the salted rim.
 
In popular culture
In the television series The Larry Sanders Show, the Salty Dog is the preferred drink of Artie, played by Rip Torn.
In the Elmore Leonard novel Swag, the Salty Dog is the preferred drink of characters Frank Ryan and Ernest Stickley, Jr.
In the computer game Kingdom of Loathing, the Salty Dog is a mid-grade drink made by mixing gin and grapefruit.
 
1 July 1947, Brownsville (TX) Herald, pg. 8, cols. 1-2:
Writer Gambles With Future
College Degree; Gives Recipes
For Cooling, Soothing Drinks
By Jack Rutledge
Associated Press
(...)
This is for those lost souls who drink—pardon the expression—alcoholic beverages and may bring down the wrath of the dries and end forever our chances of getting a degree from a certain Texas University.
 
But after all, if the aforementioned lost souls insist on drinking, we may as well give them something soothing and comparatively healthy, like the Salty Dog. On the other hand, we may even do some good. Those who try the Tepache may quit drinking entirely.
 
The recipes ,result of intensive scientific research followed by the reaction known among the lower classes as hangovers:
 
The Salty Dog.
Jigger of gin in glass of cracked ice. Fill up with grapefruit juice and salt to taste. Recommended as Summer drink for it is thirst quencher and has health angle in that it replaces salt sweated out in torrid Texas summers.
       
Trader Vic’s Kitchen Kibitzer
Garden City, NY: Doubleday
1952
Pg. 48:
Something refreshing and good for a small group can be mixed in large water pitchers. Half grapefruit juice and vodka has been dubbed “Salty Dog.” You can use canned grapefruit juice and freshen the flavor with some fresh grapefruit juice. A recent California concoction has been given the appellation “Screwdriver”—orange juice and vodka. Both should be handled cautiously because the citrus-fruit flavor can cover the taste of the vodka and before you know it you’re carrying a large-sized load.
Pg. 178: 
Out here in California vodka seems to be gaining popularity. Mixed with fresh or properly cut frozen citrus juices, you have a very palatable drink, but be wary of them. They go down so easy and taste so good that you’ll find yourself picking yourself up off the floor before you realize it.
 
SCREWDRIVER
1 1.2 ounces vodka
Fresh or frozen orange juice
Put cracked ice in a 10-ounce glass, add vodka, fill glass with orange juice.  Stir and serve.
 
SALTY DOG
1 1/2 ounces vodka
Fresh, frozen, or canned grapefruit juice
Put cracked ice in a 10-ounce glass; add vodka, fill glass with grapefruit juice.  Stir and serve.
   
2 January 1952, Anderson (IN) Herald, pg. 4, col. 5:
Mrs. A. Mitchell Palmer, widow of Woodrow Wilson’s attorney general, gives her recipe for a salty dog which, she claims, will cure laryngitis…three-quarters of a tumbler full of gin, a quarter of a tumbler of grapefruit juice and a tablespoon of salt. Mrs. Palmer says she took one and when she came to she didn’t have a sore throat.
 
18 July 1952, Charleston (WV) Gazette, Inez Robb column, pg. 6, cols. 3-4:
This is a wonderful country, full of democracy, and anyone can go to a political convention. That explains how George Jessel, a professional Democrat, happened to be in Chicago during the Republican stampede.
(...)
George was fortifying himself against Republicans with a strange potion out of a champagne glass. he let me have a taste, and after bystanders had turned the fire extinguisher on me and successfully resorted to resuscitation, he explained, that this drink was a Salty Dog.
 
“It is a little something of my own invention,” he said. “Just half fresh grapefruit juice, half vodka and a dash of salt, and you think any Democrat can win.”
 
4 December 1953, Oneonta (NY) Star, pg. 11, col. 3:
Fay, who hails from good ole Kaintucky, via California, tells me that a “Salty Dog” is quite a concoction…Gin, grapefruit juice and a dash of salt. It seems that the salt cuts the grapefruit juice and the grapefruit cuts the gin, and the…well you can go on from there.