A “kitty” can mean a kitten, but it can also mean a money pool. A “kitty” riddle is:
 
Q: What happened when the cat swallowed a coin?
A: There was some money in the kitty!
 
The “kitty” pun has been cited in print since at least 1990.
 
     
Wiktionary: kitty
kitty (plural kitties)
1. (informal) A kitten or young cat.
2. (childish, sometimes capitalized) A pet name for a cat.
3. A money pool, as for a card game, or for shared expenses.
4. (poker, slang) In a home game, a small, specified amount taken from each pot to pay the host’s expenses.
         
15 November 1990, The Monitor (McAllen, TX), “Back to School is theme of Weslaco OED program,” pg. 6D, col. 6:
Golda Roberts added a light touch by asking several conundrums, such as “What happened when the cat swallowed a penny?” Answer: There was money in the kitty.
   
Google Books
Father Christmas “It’s a Bloomin’ Joke” Book
By Raymond Briggs
London, UK: Puffin
1994
Pg. 84:
What happened when the cat swallowed some coins?
At last there was some money in the kitty.
 
Google Groups: alt.fan.prettyboy
Kitty Litter
the defender NP-t
7/7/98
(...)
What happened when the cat swallowed a one pound coin?
There was money in the kitty
     
Google Books 
The Silly Little Book of Animal Jokes
By Kindersley Dorling
Bath, UK: Mustard
1999
Pg. ?:
What happened when the pussy swallowed a penny?
There was money in the kitty.
 
Google Groups: rec.humor.jewish
“Puns of the Weak” 11/17/00
Stan Kegel
11/19/00
(...)
What happened when the pussy swallowed a penny? There was a little money in the kitty.  (Daily Groaner)
     
9 January 2001, Rocky Mountain News (Denver, CO), “Spotlite,” pg. 2D:
What happened when the cat swallowed a penny? There was a little money in the kitty.
 
Google Groups: alt.humor.puns
Puns of the Weak 8/10/01
Stan Kegel
8/11/01
(...)
What happened when the cat swallowed a coin? There was some money in the kitty! (Gail S. Angel)
   
Google Books
Penguin Pocket Jokes
By David Pickering
London, UK: Penguin Books
2006
Pg. ?:
What happened when the cat swallowed a coin?
There was some money in the kitty!
 
Twitter
David
@Davidwhufc1
What happened when the cat swallowed a coin? There was money in the kitty.
7:28 PM - 28 Jan 2011
   
Twitter
mia
@mialaffy
what happened when the pussy swallowed a penny?
there was money in the kitty
4:19 PM - 27 Feb 2011
   
Google Books
Horrid Henry’s Jumbo Joke Book
By Francesca Simon
Illustrated by Tony Ross
London, UK: Hachette Children’s Group
2015
Pg. 15:
What happened when Fluffy swallowed a pound?
There was money in the kitty.
 
Twitter
Lucky Score
@LuckyScoreNet
Why happened when the cat swallowed a coin
?
There was some money in the kitty !
#Joke from #CatJokes in #Jokes,  #Humor and #Satire
2:50 AM - 2 Jul 2017
 
Twitter
lorraine Jo King
@LahLahBean
#wednesdaywit What happened when the cat swallowed a coin? There was some money in the kitty! ☺
2:51 AM - 7 Mar 2018