Congrats to the owner and team for Mystik Dan, the winner of the 150th Kentucky Derby, by a nose.
These are wildebeest, actually: large African antelopes of the family Bovidae.
From an updated issue of the 1926-27 London Pictorial definitive series (the first series of stamps were printed in London, thereafter by South Africa government printers in Pretoria)
Issued Jan. 1950
Perf. 15×14 | Screened rotogravure | Afr. & Eng. inscriptions for South Africa | Wmk Multiple Springbok heads
SG120 13 | 1 shilling | Brown & chalky blue | Black and blue wildebeest
[Source: 2016 Stanley Gibbons Stamp Catalogue- Commonwealth & British Empire Stamps]