“Income tax” sounds like “in come Tax.” A joke was printed in a 1917 newspaper:
 
“A prosperous farmer of this county, who was looking after business in town yesterday, informed us that he had a dog named Tax and that every time he opened the door ‘Income Tax.’”
   
“There once was a dog named Tax.  I opened the door and income Tax” was posted on Twitter on May 27, 2009.
   
             
28 December 1917, Neosho (MO) Daily Democrat, pg. 3, col. 2:   
A prosperous farmer of this county, who was looking after business in town yesterday, informed us that he had a dog named Tax and that every time he opened the door “Income Tax.”
     
29 December 1947, Neosho (MO) Daily Democrat, pg. 4, col. 5:   
A prosperous farmer of this district informs us that he had a dog named Tax and that every time he opened the door, “Income Tax.” (Prosperous from corn?).
     
24 January 1960, South China Sunday Post-Herald (Hong Kong), “Income tax,” pg. 18, col. 1:
One day the teacher at school said, “I want you to write a little story about income tax.” After five minutes, she asked Peter to read his story—“I have a dog named Tax. I open the door, and in come Tax.”
 
31 July 1971, The Morning News (Wilmington, DE), Betty Boroughs column, pg. 11, col. 1:
“You have no doubt heard of the folks who owned a dog named Tax, so they opened the door and income Tax.”
 
Google Books
September 1979, Boys’ Life, “Think & Grin,” pg. 82, col. 1:
Dave: Did you hear the one about the dog named Tax?
Dan: No.
Dave: Every time the master would open the door, in come Tax.—David Lee. North Riverside, Ill.
 
Google Book
April 2007, Boys’ Life, “Think & Grin,” pg. 61, col. 1:
I once had a dog named Tax. It was easy getting him: I just opened the door and income Tax.
William Odom, Murfreesboro, Tenn.
   
Twitter
James Jones
@Jamesismtvone
@dannymcfly there once was a dog named tax. o opened the door and income tax
6:39 AM - 18 May 2009
 
Twitter
Zen Jar
@zenjar
There once was a dog named Tax.  I opened the door and income Tax. (via #zenjar )
3:08 AM - 27 May 2009
     
Reddit—Jokes
I once had a dog named Tax…
submitted May 21, 2015 by elmarko44
Every time I opened the front door, Income Tax!
 
Reddit—Jokes
Dog named Tax
submitted March 24, 2018 by tminima
There once was a dog named Tax. I opened the door and income Tax.
   
Facebook
New Life Tax Resolution
November 13, 2017 ·
“I once had a dog named Tax. I opened the door, and in come Tax.” -Worst tax joke ever.
 
Twitter   
lorraine Jo King
@LahLahBean
#1PUN There once was a dog named Tax. I opened the door and income Tax.
7:54 AM - 29 Mar 2018