The large lettering on the sign reads: "Entering Wild Horse Plains".
Across the bottom are the words:
where mountains meet river and wild horses wintered
However, a local historian,
Maurice Helterline, in studying old newspapers, history books, microfiche records, and pretty much everything he could get his hands on, has yet to come across a reference in which the town, or the area, was officially named "Wild Horse Plains".
When Europeans first came to this area the natives were using the plains nearby for pasture for their horses and the area came to be known as "Horse Plains". Somehow, sometime, someone added the word "Wild" to that. Officially though, the area and the town were variously known as Horse Plain, then Horse Plains, then just Plains, but never Wild Horse Plains. That is simply a bit of embellished local folklore.
The only thing on the sign that's factually correct is that the mountains do, indeed, meet the river. The horses weren't wild, they belonged to the natives of the area.