The horse won the 2015 National in tremendous style - he lost his life, collapsing past the finishing-line after winning the Cotswold Chase at Cheltenham in January.
Mr Hemmings said: "Many Clouds gave a lot of pleasure to a lot of people.
"He was a special horse and we will never forget him."
In all, Many Clouds won 12 races in an outstanding career.
Mr Hemmings was present at the ceremony along with Many Clouds' trainer Oliver Sherwood, jockey Leighton Aspell and his devoted groom Chris Jerdin.
The 10-year-old Many Clouds died of a pulmonary haemorrhage after the Cheltenham race.