“Why are horses always so fit?”/“They’re on a stable diet.”

A popular joke about a “stable diet” is:
 
Q: Why are most horses so slim?
A: They are on a stable diet.
 
This joke was cited on Twitter on December 30, 2012. The “stable diet” pun/joke has had several forms. In 1924, it was about eating like a horse:
 
“No, Dora, people on stable diets are not supposed to eat like horses.”
 
In 2006, it was about eating a horse:
 
“During WW2, a lot of people’s stable diet was horse meat!”
 
In 2007, it was about eating what horses eat:
 
“My doctor’s put me on a stable diet – hay and oats three times a day.”
 
     
3 October 1924, San Diego (CA) Evening Tribune, “Sunshine Pellets” by Dr. W. F. Thomson, pg. 4, col. 3:
No, Dora, people on stable diets are not supposed to eat like horses.
 
Google News Archive
26 May 1961, The Southeast Missourian (Cape Girardeau, MO), pg. 1, col. 4:
Little Liz Says:
A stable diet doesn’t guarantee that the guy eating it will have good horse sense.
   
Google Groups: demon.local
15 June Puns
nemo
6/15/06
(...)
Oats: A horse’s stable diet!
 
Google Groups: alt.humor.puns
16 October Puns
nemo
10/17/06
(...)
During WW2, a lot of people’s stable diet was horse meat!
   
Google Books
Man Walks into a Bar:
Over 6,000 of the Most Hilarious Jokes, Funniest Insults and Gut-Busting One-Liners
By Stephen Arnott and Mike Haskins  
Berkeley, CA: Ulysses Press
2007
Pg. 168:
My doctor’s put me on a stable diet – hay and oats three times a day.
 
Google Books
How to Double the Meaning of Life
By Anil
Xlibris Corporation (Xlibris.com.au)
2011
Pg. 27:
a stable diet hay and oats
 
Twitter
Hodoug
@dougigem
Why are most horses so slim?
Because they are on a stable diet.
12:26 PM - 30 Dec 2012
 
Twitter
Pet Journals
@MyPetJournal
Pathway Horse Joke of the Day!
Q: Why are most horses in shape?
A: Because they are on a stable diet. http://fb.me/LbzXLEgg
10:26 AM - 29 May 2013
 
Google Books
Gigglers: Sporty Jokes
By Toby Reynolds
London: Scholastic UK
2016
Pg. 58:
Q. Why are racehorses so fit and healthy?
A. Because they are on a stable diet!
 
Twitter
Godd Howard
@SophiaVenneri
Why are horses always so fit? They’re on a stable diet. #Jokes #Terrible
1:03 AM - 12 Jun 2016