A slur for people who lash out uncontrollably, often (always) without motive or provocation.
Typically, when one of these people begin attacking others for no reason, one can (and should) respond by loudly and clearly declaring a "Wild Animal Alert".
Someone or something that bites your ankles.
To a postman, an ankle biter is often known as a dog.
To an adult, an ankle biter may be a toddler.
To hikers, an ankle biter is sometimes a tick.
And so on.