I've seen this trope in quite a few pieces of medieval themed media, most recently medieval 2, where some knights would be wearing what essentially amounts to a chainmail dress. Is this sort of ankle low mail armor historical or a purely fictional creation?
It would be appreciated if you could post a picture of exactly what you mean.
I can say that "ankle-length" mail never existed in medieval Europe. Hauberks from the medieval era could extend as far down as the knees. Generally, the later you go in period the shorter hauberks became. In the early period there was no extra mail protection specifically for the legs, so extending the hauberk to cover the thighs made sense. Once mail leggings were invented, there was a strong incentive to shorten mail coats/shirts in order to reduce the weight carried on the shoulders or sword belt.
A mail coat, or hauberk, would generally have placed all of its weight on a user's shoulders. The further down you extended the mail to cover the legs the greater this burden would have become. You could reduce the load on the shoulders by wearing a cinched belt above the mail at waist or hip level, and this is what was done historically, but it was not ideal. The weight of the lower skirts would constantly drag and pull the belt down, and the belt itself would not have totally taken the weight of the lower hauberk off the shoulders. So extending the mail to the ankles would have made a hauberk unwearable.
Separate mail leggings were in use by the middle of the 13th century. These would have been secured to special undergarments that distributed their weight better than anything that would have been possible with a hauberk and belt.
In the era of plate armor it was common to wear what might be called a mini-skirt of mail underneath the faulds and/or tassets of a plate harness. These were for providing extra protection to the groin though, and would not have extended any further than the mid-thigh at most.
Here are two videos that talk about how leg armor was worn in the later medieval period. They do not specifically address mail armor, but the principals of loading/distributing the weight of the armor for the lower body in the 14th/15th centuries were probably similar or even identical to the methods used in say the 12th or 13th centuries (the so-called "age of mail").
https://www.youtube.com/watch?v=1otNlYgX4eA
https://www.youtube.com/watch?v=dMdNbPQipB0
P.S. It was common for surcoats worn over mail to extend past the level of the knees. But surcoats were made of cloth not mail.