I dare say any number of horses have been called a few ‘choice’ names in their time but, ‘officially’, Weatherbys – which administers horse racing under contract from the governing body, known as the British Horseracing Authority (BHA) – has various rules regarding what you can, and can’t, call a racehorse.
Each racehorse must be registered with a unique name or, in other words, a name that is not on the ‘protected’ list, e.g. Frankel, or registered to another horse. The name cannot even sound the same, or similar, to one on the protected list or one registered to another horse within the last ten years. Beyond that, the name must start with a letter and contain no more than 18 characters, including spaces, no punctuation marks, except apostrophes, and no more than seven syllables.
Vulgarity is frowned upon by Weatherbys, as is any name that may cause offence, to anyone, or confusion, in the day-to-day administration of horse racing or betting on the sport. If you want to name a horse after a living person, or one who has been dead for less than 50 years, you need to seek permission from that person, or their family. Even if a particular name is listed as ‘available’ it is still subject to approval by the BHA and will, almost certainly, be rejected if it contravenes any of the naming rules.