My one is "This bloody bus driver just knocked me off my bike," which is funny, honestly, as it infers that the horse was cycling through London because it's wearing a fluorescent bib. The horse's four-wheeled, mouth-steered bicycle is not pictured because it's still under the front wheels of the bus.
Anyway, the backstory to the hilarity is that a man was sick on a bus, and the nearest police was one of the special horsey police they save for tourists and anti-capitalist riots, and while the police officer was on the bus his loyal mount there, Invictor, decided to have a look to see if there was anything he could do to help, seeing as he's strong.
A passer-by took a photo and is now the proud owner of over 700 retweets. [Standard]
Image credit: Simon Crowcroft