My police agency seized cash and I filed my petition with notice to the person who was in possession of the cash. Of course, now someone else says they were the owner of the cash and have filed a counter-claim in my lawsuit. I believe that is improper and they should have filed an intervention. I didn't know if there were special rules for in rem proceedings. Any thoughts?

You are correct that the 3rd party claimant has miscatagorized his pleading. It is an intervention. You can ask the court to dismiss the counter-claim and force the claimant to file the proper pleading.

I agree. The claimant is really attempting to intervene in the case. He should file a plea in intervention. In either case, he has to allege a justiciable interest in the property made the basis of the forfeiture. Looks like this person is out to get whatever crumbs are left behind or force you to settle by wasting your time. Prepare your discovery requests and have fun with it. Remember that the "innocent owner" defense is an affirmative defense that must be specifically pled. If he doesn't do it, let it get to trial and object to any evidence of him being an innocent owner on the basis that the pleadings don't support it.