Old overworld sprites showing up

1) Is your issue a bug, or is it a crash?

2) If it’s a crash, please copy and paste the full error message below, or post a screenshot of it. Otherwise, leave this field blank.
See the attachment.

3) Please describe in as much detail as possible how to reproduce the bug or crash.
Get lucky to find a random old sprite. It works well at the Stable and/or at the Arena. It can also show up in realms.

4) What operating system are you playing the game on? Windows, Mac OS, Android, or iOS?
Windows 8, 64-bit.

5) What game version are you playing? You can find the version number on the title screen in the lower left corner.

6) Any other details to help us solve the problem, such as what you were doing when the bug/crash occurred, or any hints on how we can reproduce it.
Some old sprites are still present in the game (only the overworld versions, though). I really don’t like them and absolutely prefer the actual art style and I was wondering what the hell was that thing when I saw one. They aren’t colourful enough for my taste and I also don’t like their shapes.
I actually have never played with the old art style, but I know how it looked like since it’s showing on the Steam Store screenshots.