A Christmas riddle is:
 
Q: What is a dinosaur’s least favorite reindeer?
A: Comet.
   
The Christmas joke was posted on Twitter on June 11, 2015.  The joke has been printed on several images, such as this cartoon from The Oatmeal.
   
       
Wikipedia: Cretaceous–Paleogene extinction event 
The Cretaceous–Paleogene (K–Pg) extinction event, also known as the Cretaceous–Tertiary (K–T) extinction, was a mass extinction of some three-quarters of the plant and animal species on Earth that occurred over a geologically short period of time, approximately 66 million years ago. With the exception of some ectothermic species such as the leatherback sea turtle and crocodiles, no tetrapods weighing more than 25 kilograms (55 lb) survived. It marked the end of the Cretaceous period and with it, the entire Mesozoic Era, opening the Cenozoic Era that continues today.

In the geologic record, the K–Pg event is marked by a thin layer of sediment called the K–Pg boundary, which can be found throughout the world in marine and terrestrial rocks. The boundary clay shows high levels of the metal iridium, which is rare in the Earth’s crust, but abundant in asteroids.
 
As originally proposed in 1980 by a team of scientists led by Luis Alvarez and Walter Alvarez, it is now generally thought that the K–Pg extinction was caused by a massive comet or asteroid impact, estimated to be 10 km (6.2 mi) to 15 km (9.3 mi) wide, 66 million years ago and its catastrophic effects on the global environment, including a lingering impact winter that made it impossible for plants and plankton to carry out photosynthesis.
 
Twitter   
Joel‏
@BillTheRobot
What is a dinosaur’s least favourite reindeer?
Comet
*Tumble Weed*
2:21 PM - 11 Jun 2015
   
Twitter
SkyDancer‏
@TrueSkyDancer
What’s the least favorite reindeer of a dinosaur?
Comet!
3:37 PM - 11 Jun 2015
   
Reddit—Jokes
What is a dinosaur’s least favorite reindeer?
submitted August 1, 2015 by intelligilliterate
Comet!!
COMMENTS
[deleted]
Comets are totally different than asteroids but A for effort.
 
Twitter
Samantha H 🤗 🤗‏
@lifeandbooks15
Here have a christmas pun:
Whats a dinosaurs least favorite reindeer?
Comet.
🎇🐲🌌🎄
12:35 PM - 14 Nov 2015
   
Twitter
Nikesh Mistry‏
@nikmistry91
What is a dinosaurs least favourite reindeer? 😛
Comet! 😝😂
#Christmas #jokes #funny #RT #fitfam
1:50 PM - 19 Nov 2015
   
Reddit—Jokes 
what is a dinosaur’s least favorite reindeer?
submitted November 20, 2015 by HanSolosBumHole
Comet
   
Reddit—Dad Jokes
What is a dinosaur’s least favorite reindeer? (self.dadjokes)
submitted December 11, 2015 by talkingbrownies
Comet!
 
Reddit—Jokes
What’s a dinosaur’s least favourite reindeer?
submitted June 26, 2017 by GimpMaster3000
Comet.
 
Twitter
Paul Guenette‏
@GuenettePaul
Merry Christmas!! Which is the dinosaurs’ least favorite of Santa’s reindeer? Comet!
4:51 PM - 22 Dec 2017 from Bethesda, MD
 
Reddit—Jokes
Why don’t dinosaurs celebrate Christmas?
submitted December 23, 2017 by hagdog007
They didn’t like it the last time comet came.