Is the erstwhile owner making any effort to recover the dog? If not, the question is moot. If yes, counterclaim for all your costs caring for...

Sue the "mechanic" in small claims for breach of contract and/or conversion.

A report from AC that the dogs were inside and well-cared for would be good to have.

Moving to insurance, as that is the issue.

You have no actual proof and no documented damages. On these facts, you have no lawsuit.

Report the dog as abandoned to Animal Control and begin the process to keep it.

You paid for something you were not given full ownership of. Sue the seller for breach of contract.

Document the infestation with photos, then follow Attorney Taylor's advice.

Moving to debt collection, so you get better answers.

You need to file a written theft report with the police (who will do nothing) and animal control, then file a replevin action against your ex-bf...