To become substantially more complicated, especially when this was unforeseen. Colloquial expression, primarily used in (British) English. Origin: by analogy to human biological development in the womb.
The project seemed simple at first, but now it's starting to grow arms and legs.
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.