'Familiar spirits' - 'familiars'

 witch with a black cat familiar at her feet

In European folklore and folk-belief of the Medieval and 
Early Modern periods, 'familiar spirits' (or 'familiars') were supernatural 
entities believed to assist witches and cunning folk in their practice of magic.
When they served witches, they were often thought to be malevolent, while 
when working for cunning-folk they were often thought of as benevolent 
(although there was some ambiguity in both cases). The former were 
often categorised as demons, while the latter were more commonly 
thought of and described as fairies. The main purpose of familiars 
is to serve the witch or young witch, providing protection 
for him/her as they come into their new powers

Read more about familiars here

A witch and her familiar spirits taken from a publication 
that dealt with the witch trials of Elizabeth Stile, Mother Dutten,
 Mother Devell and Mother Margaret in Windsor, 1579

Frontispiece from the witch hunter Matthew Hopkins'  
'The Discovery of Witches', 1647, showing witches 
identifying their familiar spirits