Help! Unknown Devices on Wifi

Unless you reserve an IP address for a specific MAC address there is no guarantee that any device will have the same IP address each time it connects.

Some devices will randomize it's MAC address as a way of maintaining privacy so networks can't track it. There is usually a setting to control this feature and it's normally only useful for connecting to public WiFi. It's possible one of your known devices is doing this.

The name and device type detected by the router isn't 100% accurate and depends on how forthcoming the device is at report it's identity. I have several smart devices that show up as the manufacturing company of the WiFi chip and has nothing to do with the brand or model of the device.

The only 100% way of finding out whether the mystery device is yours or not is to change your WiFi password and see what doesn't work after you update all the known devices.