A Yeti or “Abominable Snowman” riddle is:

Q: What do you call a snowman with a six pack?
A: An abdominal snowman.

“Abominable” means something very bad or unpleasant—an abomination. “Abdominals” are the stomach muscles. The “Abdominal Snowman” pun has been cited in print since at least 2006.


Wikipedia: Yeti
In the folklore of Nepal, the Yeti (/ˈjɛti/) or Abominable Snowman (Nepali: हिममानव himamānav, lit. “snow man") is an ape-like entity, taller than an average human, that is said to inhabit the Himalayan region of Nepal, Bhutan, and Tibet. The names Yeti and Meh-Teh are commonly used by the people indigenous to the region, and are part of their history and mythology. Stories of the Yeti first emerged as a facet of Western popular culture in the 19th century.

13 February 2006, The Daily Mirror (London, UK), “Giggle: Greg’s giggle” edited by Fiona Parker, pg. 41:
Q: What do you call a yeti who does 100 sit-ups a day?
A: The Abdominal Snowman’

Twitter
Tog was a crybaby before it was cool‏
@Tictactoetags
What do you call a yeti that does a lot of crunches? AN ABDOMINAL SNOWMAN.
4:26 PM - 27 Jun 2009

Twitter
Andy Gunton‏
@andymooseman
Q: What do you call a snowman with a six pack?  A: The abdominal snowman. Boom, boom, as Basil would say.
4:37 AM - 19 Dec 2010

Twitter
Michael I Ferreira‏
@Michnaas
What do you call a monster in the Himalayas that has a sixpack? The Abdominal Snowman. #lamejoke
11:08 AM - 18 Dec 2011

Reddit—Jokes
What do you call a yeti with a six-pack?
submitted April 15, 2014 by [deleted]
An abdominal snowman.

4 September 2014, Bendigo (Victoria) Advertiser, “Good morning Bendigo - 05.09.2014” by Adam Holmes:
What do you call a snowman with a six pack?
An abdominal snowman.

Google Books
Gigglers: Christmas Jokes
By Toby Reynolds
London, UK: Scholastic Fiction
2015
Pg. 64:
Q. What do you call a snowman with a six pack?
A. An abdominal snowman!

Google Books
Dick and Dom’s Christmas Jokes, Nuts and Stuffing!
By Dominic Wood and Richard McCourt
London, UK: Macmillan Children’s Books
2015
Pg. ?:
What do you call a snowman with a six-pack?
The abdominal snowman.

Reddit—3amjokes
What do you call a yeti that does core workouts? self.3amjokes\
Submitted March 9, 2015 by Poppy_bagle
The abdominal snowman.

Reddit—Puns
What do you call a yeti with a strong core? (self.puns)
submitted May 15, 2015 by hayesr
An abdominal snowman!

Explain the Joke
Workout Yeti
Posted on December 17, 2015 by stfleming
Q: What do you call a snowman with six-pack abs?
A: The Abdominal Snowman!
(...)
Abominable means something that is very bad, unpleasant, something that causes revulsion.  Abominable (no d) sounds a lot like abdominal (has the letter d) and remember, abdominal refers to the stomach.

My Town Tutors
School Jokes: Snow Day Jokes
Posted on October 31, 2016 by Marilyn D\’Agostino
(...)
17. What do you call a snowman with a six pack?… An abdominal snowman.

Reddit—Jokes
What did they call Frosty after he got shredded at the gym?
submitted December 15, 2017 by adamtwosleeves
The abdominal snowman

Twitter
Shit Jokes‏
@ShitJokes
What do you call a snowman with a six pack?
An abdominal snowman.
2:16 PM - 26 Feb 2018