I was looking at page after page of thermometer probes on thermoworks.com and noticed that the very last probe on the very last page is a rectal probe. They say it's for use on rats. OK, if they say so.
www.thermoworks.com

Mini Rectal Probe Type T, 0.1°C (RET-2)
Specializing in cool, unique and professional temperature tools. Super-Fast thermocouples, thermistors, infrared thermometers, data logging and more.