Comment by redler

An unfortunate but common sequence of events, where this:

We never signed any contracts or work-for-hire agreements

...leads to these:

The agency told me that I could publish the work as my own for the "exposure"

I never even got paid the peanuts they owed me

The agency responded by threatening me with legal action and worse

Whenever a client states or implies that "the exposure" will be payment enough, alarm bells should be ringing.