Why was the name "arange" chosen for the numpy function?

It is explicitly modelled on the Python range function. The precedent for prefixing a was that in Python 1 there was already a variant of range called xrange.

NumPy derives from an older python library called Numeric (in fact, the first array object built for python). The arange function dates back to this library, and its etymology is detailed in its manual:

arrayrange()

The arrayrange() function is similar to the range() function in Python, except that it returns an array as opposed to a list.

...

arange() is a shorthand for arrayrange().

• Numeric Manual
  2001 (p. 18-19), 1999 (p.23)

Tellingly, there is another example with array(range(25)) (p.16), which is functionally the same as arrayrange().