The drawings, by renowned artist Laurence Broderick, were discovered almost two years after he died aged 88.
They were stashed under a bed at his family’s home in Cambridgeshire.
Broderick specialised in figurative carvings in stone and castings in bronze, often with a nature theme.
He was best known for his six tonne, 4.5m long bronze bull sculpture - named one of the world's top ten public artworks.
The Bull was erected in 2003 at the Bullring shopping centre in the centre of Birmingham.
The portfolio of 14 signed drawings sold for a total of £3,380 including buyer premiums at Batemans Auctioneers in Stamford, Lincs.
Auctioneer Stephen Drake stumbled across the unseen sketches while assessing the value of the family’s estate following the death of Broderick’s wife Ingrid last August.
He said: “I was visiting the family home to assess other related works.
“We knew there would be many interesting items and really hoped we would find something relating to the Birmingham Bull.
“It was a fantastic moment when the incredible portfolio of his work was pulled from under the bed.
"I have done the job of an auctioneer and valuer for over 27 years and its moments like this that make my job the best one in the world.
"The family have decided that these items need to be shared and not kept under the bed or in a loft anymore.”
In the portfolio are the original sketches for the bull, complete with handwritten notes by Broderick about his favoured design.
The collection, comprising 14 signed A2 sketches and dated July 2001, were scribbled two years before the The Bull was unveiled.
On the final sheet, Broderick wrote: "14th drawing and it was the drawing I decided to make."
The bronze, officially titled The Guardian, was erected in 2003, as the focal point of the city's shiny new Bullring shopping centre.
At the time, Broderick described it as a "gateway emblem for Birmingham throughout history".
He added: "The objective of the sculpture is to reflect the characteristics of the trade and the market place that are synonymous with the bull.”
The work would ultimately go on to inspire the city's raging bull, later named Ozzy, which thousands of people turned out to visit after it featured in the 2022 Commonwealth Games.
Broderick famously became embroiled in a bitter £5 million court battle in 2022 with his two sons when he cut them out of his will and left everything to the National Trust.
He disinherited his sons Graeham and Roger after they blamed him for the death of their younger brother Ollie in 2019.
After Broderick was “blamed” for Ollie's death by his other two sons, he cut them out of his life and froze Graeham out of the family business, London's High Court heard.
Graeham then sued his father, claiming he was owed up to £5 million by his parents because he had been an equal member of a business “partnership” with his mother and father for 20 years.
The court case was eventually halted after the father and son agreed a settlement.
5. The Birmingham Bull
The Guardian statue outside of the Bullring shopping centre in Birmingham. The six tonne animal, twice the size of a real life bull was created by Laurence Broderick and erected in 2003. Photo: Jason Wells