First of all, make sure you're watching "The Hippopotamus" from 2017. There are at least 3 other films titled "Hippopotamus" from 2015 and 2018. The teenage boy who has intercourse with a horse is played by Tommy Knight, who would be known to Doctor Who fans as the character Luke Smith. (He was actually around 23 or 24 at the time.)
The scene in question begins near the end of the movie, at about 1:14:50 and only lasts 5 or 6 seconds.
The movie is available on Tubi, the Roku channel, and Pluto TV. I checked the versions on Tubi and Roku and both have the scene. The Tubi version has better picture and sound quality. I wasn't able to check the version on Pluto because every time I tried to fast forward I got a new set of ads. It's also available for rental on Amazon, Vudu, Google Play, YouTube, and Apple TV, but I haven't checked any of those versions.