This one is a perfect example of somewhere your DnD character does NOT want to take their horse. Not just riding, but you don't want to try to walk it through there either. All those holes and shadows that look like holes will all look like holes to your horse, and even if your horse is trained so they won't freak out in a forest (they freak out in forests because, basically, they have one eye on each side of their head) they might baulk at going anywhere near them. The other possibility (the worse one) is that they try to jump over them onto what they think is solid ground, but almost certainly isn't, at which point they will almost assuredly break either their ankle or their leg.
|"That doesn't look so bad" - someone who doesn't ride horses|
If your character is looking at this, and they have a horse, they'll have to either find a way around or take their horse back and leave it somewhere. Unless neither the player nor the character have any experience with horses (this is pretty basic), in which case they can try to make their horse (not them) roll a wisdom save (determination) to go through it, and then a dexterity save to not break at least one leg (based on how badly they failed the save). The horse is going to have to keep making that dex save. How often depends on the ground, but potentially as often as every 5 minutes.