Freezing like that is usually caused by a stuck sim. Use Master controller to reset your town. You don't have to have a town hall you can do it from a computer. Just click on the computer and choose the NRAAS Menu and you can acess master controller and all of its options there. Reset town is under the Town menu. Just do not unpause your game before you reset everything. If it is a stuck sim then it should fix it but if there is an issue causing it it will likely occur again. Wild horses, the ones generated by the game, often get stuck and cause this issue. I use NRAAS register to remove them and stop the game from making more. I had this problem a few months ago and ended up having to make a new save because the issue kept happening.
"Vexlings mimic their surroundings by creating intricate markings on their coats to match the character of the landscape they call home. Interestingly, they seem to have individual tastes- while some are painted bright floral patterns, others choose more subtle forest tones. How they decide the shape and color of their markings is still unknown, as are most things about this elusive species, as they have proven difficult to document and even harder to study. They are typically fairly small in stature, have slightly elongated and pointed ears, and have refined, slightly dished faces, not unlike the modern Arabian horse. Their eyes are typically flat black. Manes are usually either sparse or nonexistent, though they typically have full tails (useful as they are plagued by flies same as any other wild creature.) They can manipulate their mane and tail color same as their coat. Antlers on males are rare and seem to be a recessive trait.
Though their name would lead you to believe they were capable of blending completely into their surroundings, most vexlings are not so subtle- usually they choose to represent details of their home that appeal to them, such as species of flowers, the features of other animals they share a habitat with, or repeating patterns or views they see frequently. There seems to be no true pattern or inheritability to their appearance; every individual in a herd may appear completely different despite sharing an environment and a family line. The complexity of their environment does seem to have an influence- vexlings living in vibrant rainforests will have a much wider spectrum of appearance than a herd living on a foggy, sparse moor. The expression of their coats varies hugely as well, vexlings have been reported to only have small, muted markings on their faces or shoulders, while others are completely covered in vivid floral hues. Their name and reputation for being able to disappear more likely stems from their ability to move soundlessly through even the densest forests, as well as their incredible speed. It is not uncommon for a researcher to look away from a herd for a few small seconds only for the whole herd to have vanished when they look back; a skill which probably earned them their nicknames such as "treeshifters," "forest spirits," and "foglings"'.