if you mean like just on your front lawn or yard, as long as you don't have any kinds of weed killer of or chemicals it will be fine.but you also got to watch your plants to make sure there not poisonous. And its a good idea just leave them on it for a day or just half a day. We put our horses on the lawn for the day especially when it needs to be mowed. ( i could be wrong but, we do it so i think you will be fine )
Putting your horse on the lawn could hurt it by causing colic. This is because in the pastures, the horse is used to eating hay or dried grass. So, when it comes to the lawn, it feeds on fresh grass which causes the colic. You can help prevent the horse from getting colic by slowly introducing it to fresh grass.
Mikala is correct. As long as you haven't treated the lawn with pesticides or fertilizers, it's fine. The only 2 caveats are, if you like to have a lush, rich lawn and your horse is used to a pasture that is not so rich then you could have some colic problems. Also, young spring grass is higher in sugar than what you'll find during the summer so limit any time in the spring when the new grass is sprouting.