“What is a sheep’s favorite fruit?”/“A baa-nana.”

A banana riddle is:
 
Q: What is a sheep’s favorite fruit?
A: A baa-nana.
 
“What would you get if you crossed a sheep and a banana? A baa-nana” was included in a 1996 book of riddles. “Which fruit does a sheep like best?  Baa-Nana” was posted to the newsgroup soc.sexuality.spanking on April 21, 1999.
 
     
Google Books
Riddles
By Joseph Rosenbloom
Illustrated by Sanford Hoffman
New York, NY: Sterling Publishing Company, Inc.
1996
Pg. 84:
What would you get if you crossed a sheep and a banana?
A baa-nana.
 
Google Groups: soc.sexuality.spanking
New Beginnings-part 8a
SamPast
4/21/99
(...)
One joke read: Which fruit does a sheep like best?  Baa-Nana.
 
Google Books
Blonde Walks into a Bar:
The 4,000 Most Hilarious, Gut-Busting Gags, One-Liners and Jokes
By Jonathan Swan
Berkeley, CA: Ulysses Press
2008
Pg. ?:
What’s a sheep’s favorite fruit?
A baanana!
 
Twitter
Dawn Prickett
‏@Nairtseuqe
7 yr old to me “What’s a sheep’s favorite fruit?  ....... Baaaaaaaananas”
8:59 AM - 20 Jul 2009
   
Twitter 
Sarah Meow
‏@MuahahahaMeow
What is a sheep’s favorite fruit? Baanana!
10:09 PM - 2 Jan 2015
 
Facebook
Sheep jokes
May 14, 2015 ·
This joke comes from Ronan: What’s a sheep’s favorite fruit .....baaaaaaaanana
   
The Amazingly Lame Joke of the Day
Wednesday, November 16, 2016
What is a sheep’s favorite fruit?
A baaa-nana.
Posted by Tim Wycislak at 1:08 PM