In Cornwall in the South West of England, this is a dialect name for a woodlouse, so although not strictly slang it still owes its origins to dialect and common usage as slang words do. The difference between dialect and slang is that dialect is not constant, for example just across the Tamar in the county of Devon a woodlouse is ‘chickypig’ while in Somerset it’s ‘gramfy-coocher’ and in Gloucestershire ‘johnny-grump’.
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.