The past was a place of different values and ethics, and to a certain extent, you cannot judge the actions of people a thousand years ago against the moral standards we hold today. But on the other hand: YE GODS, WHY ARE YOU SHOVING EELS UP THE ANUSES OF HORSES?
Let's start from the beginning. Eels are a versatile creature, with which we've had a mixed relationship over the years: mainly that of them being our food, or something we shove up our own anuses. That last one happened in the last week by the way.
As well as this, we've occasionally used them as a sort of ball in what is clearly the worst sport of all time. Palingtrekken – eel pulling – involved taking a rope and hanging it over a canal, before attaching a live eel to the center. Next, the human participants would board boats and attempt to pull down the slippery, thrashing eel, with whoever managed it being declared the winner, and centuries later a real jerk.
Though it sounds like an obscure novelty that nobody really played, you forget how little by way of entertainment people had a few hundred years before Shrek. The people of Amsterdam, at least, took it extremely seriously and reacted badly when it was banned for being just Lex Luther levels of evil. When an illegal game of eel yank broke out on the Jordaan on July 25, 1886, the police attempted to break it up. Rather than try e.g. Chess, the crowds turned on the police, beating one of them mercilessly while his colleague fetched reinforcements to really escalate the entirely insane situation.
Instead of checking out badminton, fighting took place all evening and resumed the following day. The military got in on the fray, and before the day was out 25 people were dead, defending their right to yank on an eel for what they called sport. Side point, but imagine how this would go down with people who moan about skateboarding happening at the Olympics.
So as you can see, we aren't exactly best friends with the eel, despite having consummated our relationship on a number of occasions.
But in the middle ages and early modern period, we appear to have done something else equally as bad – sending eels up the anuses of horses in order to sell them. How does putting an eel up there affect sales price, you ask? Well, good but very unusual question there, thank you.
The practice, known as feaguing, was done to older horses that were getting a bit too long in the tooth. In order to make them appear youthful again, you could try and liven up the horse by inserting a live eel into its rectum whenever somebody came to look around the horse sales showroom. This would distress the horse, and cause it to be a lot livelier than if it e.g. didn't have a live animal thrashing around in its rectum.
The practice is referenced a number of times (helpfully compiled by the Surprised Eel Historian here), including one quote by John Milton in 1628, in which he described a type of bird as “more useful to grooms because they are by nature lively and brisk and prancing, and if they were forced into the anus of scraggly horses would make them livelier and quicker than if they had ten live eels in their bellies.”
Later on, horse sellers moved on from selling horses in this manner. But only because they realized that it was much easier to get the same effect by putting ginger up their bottoms.