Help identifying a potentiometer/resistor like symbol in the I/O stage of a microcontroller
They're probably intended to represent small transistors operating in the linear region when used as (configurable) pull-ups or pull-downs. True resistors of such high value are expensive (large acreage required) to make in many VLSI digital processes so transistors are used. See this reference. 

Because the effective resistance of such a pull-up/pull-down is not constant and because it can be switched, the symbol represents it more accurately than a simple MOSFET in series with a resistor. 

Keep in mind that commercial I/O circuitry that meets ESD and other requirements is generally fairly complex and proprietary and you can be sure that whatever you are seeing is a simplified version. See, for example, this student thesis.