'Mule killer' definitions:

Definition of 'Mule killer'

From: GCIDE

Mule killer \Mule killer\ Any of several arthropods erroneously supposed to kill live stock, in the southern United States, by stinging or by being swallowed; as: (a) A whip scorpion. [Florida] (b) A walking-stick insect. [Texas] (c) A mantis. (d) A wheel bug. [Webster 1913 Suppl.]