A pot of gold is what some people believe that a leprechaun’s keeps at the end of a rainbow. A rainbow riddle is:
   
Q: What will you actually find at the end of every rainbow?
A: The letter W.
   
“What is at the end of the rainbow? A. The letter w.” was printed in The News and Observer (Raleigh, NC) on October 28, 1973.
 
         
Wikipedia: Rainbow
A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows caused by sunlight always appear in the section of sky directly opposite the Sun.
(...)
The Irish leprechaun’s secret hiding place for his pot of gold is usually said to be at the end of the rainbow. This place is appropriately impossible to reach, because the rainbow is an optical effect which cannot be approached.
     
Newspapers.com
28 October 1973, The News and Observer (Raleigh, NC), The Mini Page, pg. 3, col. 1:
What is at the end of the rainbow?
A. The letter w.
Jimi Ann Chrisco of Raleigh
   
Newspapers.com
21 June 1979, Lexington (KY) Herald, “Just Kidding,” pg. D-16, col. 4:
What’s at the end of the rainbow? The letter w.
—Kelly Sallee, 10, Harrodsburg.
 
Google Books
Take Me to Your Liter:
Science and Math Jokes
By Charles Keller
New York, NY: Scholastic
1991  
Pg. ?:
What is at the end of the rainbow?
The letter “ W. ”
 
Newspapers.com
25 June 1999, Los Angeles (CA) Times, “The Kids’ Reading Room; Pint-Sized Punch Lines,” pg. E8, col. 3:
What’s at the end of the rainbow? The letter W. (Chris, 8, Malibu, Webster Elementary)
   
Google Books
Reading is Funny! Motivating Kids to Read with Riddles
By Dee Anderson
Chicago, IL: American Library Association
2009
Pg. 72:
What will Bear find at the end of every rainbow?
the letter “w”
 
Twitter
CHOKOLEIT
@chokoleitWiTS
At the end of every rainbow is . . . . . . . letter W ! =)
3:06 PM · Aug 13, 2010·Twitter Web Client
   
Twitter
Gemma Horton
@gedav65
W- wish…...hihihi“@chOkOLeit2010: At the end of every rainbow is . . . . . . . letter W ! =)”
3:11 PM · Aug 13, 2010 from Vinings, GA·Twitter for iPhone
   
14 December 2013, Iran Daily, (Tehran, Iran), “Riddles.” pg ?:
What is at the end of a rainbow?
The letter ‘w’
   
Reddit—Dadjokes
Posted by u/Zamibe April 13, 2016
What is at the end of a rainbow?
The letter W.
 
Newspapers.com
8 January 2017, Orlando (FL) Sentinel, “Jumble for Kids,” pg. 16, col. 4:
What at the end of a rainbow?—
The letter “w”
   
Google Books
Best Kids’ Jokes Ever! Volume 1
By Highlights
Columbus, OH: Highlights for Children
2018
Pg. 104:
What is at the end of a rainbow?
The letter W.
 
Twitter
•𝙠𝙞𝙩
@p0rknbeans_
What will you actually find at the end of every rainbow? — the letter w? HAHAHAHA
kitk1tt
CuriousCat is a Q&A social network that allows you to connect with your followers and know new people.
curiouscat.qa
8:35 PM · Feb 4, 2021·Twitter for iPhone
 
Facebook
Richard Woodcroft asked a question in One Liners And Snappy Gags.
March 5, 2021 at 2:24 PM ·
What one thing would you most definitely find at the end of every rainbow????
W