A police dog unit is called K-9 (canine). It’s not called K-10 because—according to the joke— that would be a cat (kitten). 
 
“Police dog is K-9 (canine). If cats were trained they would be K-10 (kitten)” was posted on Reddit—Shower Thoughts on September 4, 2014.
 
   
Twitter
Adrian Foyn‏
@AdrianFoyn
K.9 vs K.10
Dog vs kitten
o_O
8:58 AM - 12 Jul 2011
 
Twitter
Drei Dominique‏
@papadreiii
There’s a dog called K-9. And there’s a baby cat called K-10. Gerit? K-10? Kitten.
#libresapak
11:01 AM - 29 Jun 2013
 
Reddit—Shower Thoughts
Police dog is K-9 (canine). If cats were trained they would be K-10 (kitten)
submitted September 4, 2014 by Crakit
   
Twitter
Wopian // API‏
@Wopian00
Police dog is K-9 (canine). If cats were trained they would be K-10 (kitten) http://bit.ly/1o1ZVVX  #reddit #thoughts
12:47 PM - 4 Sep 2014
 
Funny Status
Police dog is K-9 (canine). If cats were trained they…
September 4, 2014 Animal Status Updates
Police dog is K-9 (canine). If cats were trained they would be K-10 (kitten)
 
Reddit—Shower Thoughts
Police Dog is K-9 (Canine). If Cats were trained they would be K-10 (Kitten).
submitted March 1, 2015 by IkAlfie
 
Reddit—Jokes
Why dogs are called K9
submitted August 8, 2015 by Nox1re
Why dogs are called K9? Because cats are K10
   
Twitter
@klaysrocco
Why is the dog called a k9???
Bc a cat is called a k10 (kitten) 😂😂
12:07 AM - 2 Oct 2015
 
Reddit—Jokes
Why is a police dog called a K-9?
submitted August 12, 2017 by InteriorAllelegator
Cause if it was higher it would be a cat