In case anyone doesn't know, there's a great story behind this. Years ago, someone went on the Fallen London subreddit to lament that they had made a mistake while attempting to buy some number of weasels, accidentally misclicking and purchasing 500 weasels at once.
This weasel-afflicted player wanted to know if one of the developers could undo the purchase or give them back the substantial amount of in-game money they'd unintentionally spent on all those extraneous mustelids.
One of the devs responded to the post, on their lunch break and slightly drunk. They could not undo the purchase or refund the player, but they did go in and slap together a brand new event in the game that triggers when a player comes to possess 400 or more weasels, the text of which can be seen in the original post.
Aside from being hilarious, this event also bestows a unique quality on the weasel owner, which in Fallen London is worth far more than mere in-game currency. In the years since, further evolutions of this unique quality have been introduced for owning 400 of the game's other, more expensive varieties of weasel.
Yet another reason Fallen London is one of my favourite games.