A Christmas riddle has been printed on many images:
   
Q: Which of Santa’s reindeer(s) has bad manners?
A: “Rude"olph.
 
or
 
Q: Which of Santa’s reindeer(s) needs to mind his manners the most?
A: “Rude"olph.
 
The plural of “reindeer” is either “reindeer” or “reindeers.” The joke has been cited in print since at least 1999.
   
       
Wikipedia: Rudolph the Red-Nosed Reindeer
Rudolph the Red-Nosed Reindeer is a 20th century reindeer created by Robert Lewis May. Rudolph is usually depicted as the ninth and youngest of Santa Claus’s reindeer, using his luminous red nose to lead the reindeer team and guide Santa’s sleigh on Christmas Eve. Though he initially receives ridicule for his nose as a fawn, the brightness of his nose is so powerful that it illuminates the team’s path through harsh winter weather.
 
Rudolph first appeared in a 1939 booklet written by Robert L. May and published by Montgomery Ward, the department store.
         
24 December 1999, Lincoln (NE) Journal Star, “Have yourself a merry riddle Christmas,” pg. 8B, col. 4:
Which of Santa’s reindeer has bad manners?
Rude-olph.
 
Google Groups: uwo.comp.helpdesk
Humor: [GCFL] Santa Funnies
Heisey, Tom
12/22/00
(...)
Which of Santa’s reindeers needs to mind his manners the most?
“Rude"olph
   
Google Books
The One Year Devos for Teens
By Susie Shellenberger
Wheaton, IL: Tyndale House Publishers, Inc.
2002
Pg. 359:
Which one of Santa’s reindeers needs to learn his manners?
“Rude” ‘olph.
 
Google Groups: alt.humor.puns
Puns of the Weak 12/20/02
Stan Kegel
12/21/02
(...)
Which of Santa’s reindeers needs to mind his manners the most?
“Rude"olph (Maurizio Mariotti)
   
22 December 2002, Yakima (WA) Herald-Republic, “Sarah Jenkins Column—Read, Write, Laugh and Spread Joy,” pg. E5:
13. Which of Santa’s reindeer needs to mind his manners the most?
13. “Rude"olph
     
22 December 2005, Sydney (New South Wales)) Morning Herald, “Bridge,” pg. 27:
Which of Santa’s reindeers needs to mind his manners the most? Rude-olph
     
Twitter
tammyburns
@tammyburns
Another reindeer joke for you - Which of Santa’s reindeer has bad manners?  “Rude”-olph!
9:40 AM · Dec 22, 2008·Twitter Web Client
   
Twitter
Andy Buckingham 🎄
@andybee
Replying to @matt
@matt Which of Santa’s reindeers needs to mind his manners the most? Rude-olph
6:57 PM · Dec 23, 2008·Twitter Web Client
 
Google Books
Christmas Crackers
Compiled by Matt Wilce
Lulu Press (Lulu.com)
2012
Pg. ?:
Which of Santa’s reindeer has bad manners?
Rude-olph!
 
Reddit—Jokes
Posted by u/[deleted] December 27, 2017
What do you call an impolite red-nosed reindeer?
Rude-olph.
 
Reddit—Cleanjokes
Posted by u/itsidahojoe December 19, 2019
Which reindeer is most likely to offend you?
Rude-olph