I just saw this post.
What part of the state?
I have not heard of any. I have three possibilities come to mind.
A privately owned animal that escaped.
A pronghorn with atypical horns. I consider this unlikely. Pronghorn are common in the Snake River plains in the southern part of the state and all residents are familiar with them. Pronghorns are significantly smaller than Ibex and have a distinct color pattern. Not likely a case of mistaken identification.
Elk with atypical antlers. Elk are common in the mountainous forested areas of Idaho. Closer in size and color to a real ibex. So a misidentification is possible. But then again, residents are used to seeing them. Locals would know if it was an elk or not. See my avator photo. Those are elk. Could a misshapen antler confuse someone?