There is an average of one or so thunderstorm days in October across the Dakotas and Minnesota. This is somewhat misleading because it is just an average. Some years, there may be three or four thunderstorm days, followed by years with no thunder being reported at all. Although the progression of the seasons is generally the same year after year, the actual weather varies quite a bit. Summery warm and humid conditions are certainly not as common in October as they are in July, but it is not unusual for there to be enough ingredients for thundershowers.
Severe weather in October is much more unlikely. However, severe weather is possible any time the correct ingredients in the atmosphere come together, no matter the season. Thunderstorms with damaging wind, large hail and even a tornado are possible, if extremely rare, as we progress through the month of October.