Good Afternoon and welcome! I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
All of these symptoms are neurologic in nature so I would worry about an Encephalitis, Liver disease, brain trauma or brain tumor. Your first step is to run a blood panel to rule out metabolic disease or infectious process. After that, a spinal tap and MRI would be the next step. This can only be performed in an equine facility. This appears to be very serious for Slinky so it is important to follow through with testing. Be sure to isolate Slinky until you know what is causing this.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please REPLY so I can clarify for you.
Sincerely, Dr. Loretta