I first learnt of Pot8os the horse due to a Tumbler meme shared to Facebook. The story of this horse's name has been quite popular, and has been spread widely throughout the internet, including the usual sites of Tumbler, Facebook, Pinterest, and Reddit.
The story goes like this: Willoughby Bertie, the fourth Earl of Abingdon in England, bred a thoroughbred horse (born 1773) that the stable lads called "Potatoes". When the Earl asked one of the lads to write the horse's name on its corn feed bin, the lad heard the spoken name as "Pot" "8 Os". The feed bin thus became adorned with "Potoooooooo", and the horse gained a new spelling of his name because the Earl found it hilarious.
Potoooooooo was set to racing in 1776. Entered into the first few races as Potoooooooo, his name then became shortened in spelling to simply "Pot8os". Pot8os ran successfully for many years until being retired in 1783. Many of his races were 4+ miles in length, and he was considered the best horse of his time. His notable major wins are as follows:
1200 Guineas Stakes (1778)
Clermont Cup (1779, 1780, 1782)
Jockey Club Plate (1780, 1781, 1782)
October Cup (1780)
Newmarket Whip (1781, 1783)
Craven Stakes (1782)
Upon retirement, Pot8os was put out to stud and was very popular with breeders, siring many winners at a good fee until his death at age 27 in 1800.
Pot8os carried the blood of his sire, Eclipse (born 1764), and his dam, Sportsmistress (born 1765), sired by Sportsman (born 1753). According to an article I found, you can use the website http://www.allbreedpedigree.com/ to confirm that a thoroughbred horse of this name actually existed. Further confirmation can be had on the same website by searching one of Pot8os's descendants, for instance the thoroughbred called Medoc, born 1829.