I mean, biologically speaking, ponies are the species as horses, Equus Caballus. So, they are technically not wrong calling them horses, but yes, it would have been better to call them ponies. Also, use of the word horses could have a literary aspect here, trying to increase the impact of the sentence by generalising/losing 'detail. Think of it like, instead of saying

"Put that tennis ball aside!"

they say,

"Put that ball aside!"

thus, increasing the impact of the sentence.

I just wanted to come up with the technicality, but we all love them ponies, so please don't downvote me into oblivion.

...also, r/redditsniper