He went from tumbling mustard to the "bronc they couldn't break" in
1924 the best known bronc in rodeo history was bought. tell me the
name he is more commonly known by?

Dear snegaard,

The answer is: "Five Minutes to Midnight", a black stallion. 
 
I found the following information in the book "Way Out West", by Jane
and Michael Stern, in the chapter about famous animals in the history
of rodeo:
 
"'Five Minutes to Midnight' was caught together with a herd of wild
horses in the Sacree Indian Reservation in Alberta, Canada, in 1924.
He was saved from going to the slaughterhouse by a buying agent of the
'Calgary Stampede' who intuitively bought him for five dollars. 'Five
Minutes to Midnight' has most likely been the best bucking bronc in
rodeo history. In 1926, when his career began, he was called 'Tumbling
Mustard', but he was as black as the then-famous 'Midnight' (though he
had white fetlocks at the hind legs and a white blaze on his head).
When 'Tumbling Mustard' started bucking off cowboys as successful as
'Midnight', his name was changed. 'Little Five' was only 810 lbs, has
a bushy tail and a shaggy hide, which always looked a bit messy.
During his astonishing career, which lasted up to a leg injury in
1944, he bucked off more than 2000 men. (...) His tombstone on the
Colorado ranch of his owner, Verne Elliott, bears the words: 'He was a
bronc they could not break'."
 
(Translated back to English from a German edition) 
 
Source: 
Jane and Michael Stern: "Way Out West" (German translated edition).
Ullstein publishing house, 1996. ISBN 3-550-06890-5
 
Hope this answers your question! 
Best regards, 
Scriptor

Thank you for the timely answer. The information you sent was great.
Thanks much!