“A guy walks into a bar…” is a typical form of what has been called the “bar joke.”  A marsupial version is:
   
“An Australian marsupial hops into a bar and the barman says, ‘Wallaby?’ The marsupial says, ‘I’ll have a pint, please.’”
 
“Wallaby” sounds, perhaps, like “what’ll it be.” The joke was posted on Twitter on October 9, 2010.
   
       
Wikipedia: Wallaby
A wallaby is a small- or mid-sized macropod native to Australia and New Guinea, with introduced populations in New Zealand, UK and other countries. They belong to the same taxonomic family as kangaroos and sometimes the same genus, but kangaroos are specifically categorised into the six largest species of the family. The term wallaby is an informal designation generally used for any macropod that is smaller than a kangaroo or wallaroo that has not been designated otherwise.
 
Twitter 
me
@ireland_eye
An Australian marsupial hops into a bar and the barman says, ” Wallaby? “The marsupial says, ” I’ll have a pint, please.”
6:21 AM - 9 Oct 2010
   
Twitter
BIG_BEAR_F1
@BIG_BEAR_F1
An Australian marsupial hops into a bar and the barman says, ” Wallaby? “The marsupial says, ” I’ll have a pint, please.”
7:01 AM - 9 Oct 2010
     
Twitter
Spock Thornaby 🖖
@Spock_Thornaby
An Australian marsupial hops into a bar and the barman says, “Wallaby?”
The marsupial says, “I’ll have a pint, please.”
2:55 AM - 25 Sep 2013
 
Twitter
Patrick, Probably?
@PA77ICUS
A marsupial walks into a bar in Australia, bartender says “wallaby?” #punoftheday
3:09 AM - 9 Feb 2017
 
Twitter
Terry
@BenTerrion
Replying to @ILoveTheIOM @iomnewspapers and 8 others
A marsupial walks into a bar.
Barman: Wallaby?
Marsupial: A pint of bitter please mate.
12:20 PM - 18 Jul 2018
 
Twitter
🤣 The Dad Joke Man 😉
@DadJokeMan
An Australian marsupial hops into a bar and the barman says, “Wallaby?”
The marsupial says, “I’ll have a pint, please.”
1:49 AM - 27 Feb 2019