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Polo and Polo Ponies
"What happened to Polo?"
Obviously, John Steed plays Polo. When Dr. Beardmore asked Steed in The Little Wonders about what exercises he takes, Steed mentioned that his lack of activity in the fresh air is caused by the end of the Polo season. He has a friend who owns a 'string of polo ponies' (Don't Look Behind).He buys a Polo mallet in Death of a Batman from ten pounds which he got back from his former batman and gets a present from Lady Cynthia: a white polo helmet in the same episode.
This indicates that Steed is a polo player too, he confirms it in The Murder Market when he's talking about photos showing him playing polo.

In Death of a Batman Steed is after a pair of Polo Ponies.
He's waiting for a phone call and is afraid that they are going to sell them behind his back as a "very important person" is after the ponies.

Steed was successful in buying the polo ponies as Mrs. Gale is asking a question about them in The Medicine Men. Steed starts to play (Indoor Golf), and he tells Mrs Gale that the polo ponies "took advantage of each other and lost interest in the game".
He talks one more time about them in The Murder Market, where he's telling Barbara that he has a couple of polo ponies at the stables of his riding club and he's rather attached to.

Steed's enthusiasm about Polo is obvious. There are many trophies and photos left to the kitchen door in his third flat in series 4.
Trophies and photos in Honey for the Prince
Fanart: Steed the Polo player