A BabyMetal

A_BabyMetal is a code pointer used in the normal animation for the Arachnotron.

is a wrapper around A_Chase that plays the sound effect DSBSPWLK before calling that other function. This makes the Arachnotron play its walking sound while invoking the usual monster AI.

Uses
The  code pointer appears in the following states in Doom's state table:

Example
The following is an example of how to set the  code pointer in a Dehacked file:

Pointer 21 (Frame 34) Codep Frame = 641

Or using BEX syntax:

[CODEPTR] Frame 1234 = BabyMetal