I'm reblogging this story from @HistoryWeird's blog here. The original source was The Morning Post October 5th 1866 and the Kentish Chronicle, October 8th 1866.
"In the autumn of 1866 several English newspapers reported the 'very exciting scene' of a porpoise frolicking in the Thames in central London. There were several sightings of the creature between Tower Bridge and Waterloo. According to one report:
“'It appears that for some time past this interesting visitor has been disporting itself in the ample bosom of Old Father Thames and has been much admired by the voyagers up and down the river.'
"But the porpoise’s playful antics were not tolerated for long. On Wednesday October 3rd two boats were launched from Blackfriars Road, each packed with men carrying rifles. They spent several hours firing shots into the murky Thames, in places where they thought the hapless cetacean might be swimming. They eventually got lucky and the porpoise died after being hit with several shots. A heated argument later broke out between the riflemen over who was entitled to its carcass. Another newspaper expressed relief that the sideshow was at an end and everyone could get back to work:
"'The creature of the deep having now been dispatched, the workers [of London] can now cease dallying along the Thames banks and attend to more rightful duties.'"
I can't really figure out the reason they killed it, other than that it was there. I just picture one of the men in the boat going, "Make us smile with your novelty-value, will you? We'll see about that!" *cocks gun* "Move along, people. Nothing to see here. Just a creature shot dead for being too charming and playful. We will tolerate none of that, so everyone back to work."