An 'ask' was the common name for a newt in Scotland and northern England as long ago as the 14th century. It came from Old English 'athexe' before getting worn down to 'ask'. Thereafter, people in the West Midlands area obviously decided to start building it up again by adding '-er'!
It is worth noting perhaps that 'newt' originated as a 'mistake', whereby people moved the 'n' from 'an ewt' and made it 'a newt'. In the opposite way, they changed 'a nadder' to 'an adder' (the snake). Both 'ewt' and 'newt' are variants of 'eft', which is the oldest of all the words involved here.