Yes, they did. But they did not sue under the provisions of the ESA and the agency's lack of investigation into the status of the species. They sued to demand admission of existence. A suit under the ESA is a demand to study endangerment status. It is more than obvious that either government has not looked into the phenomenon (doubtful), or they refuse to comment on it in any other than a mockingly way (more than obviously so). Thus, when private sasquatch researchers present their evidence of a dangerously low population (easily done), and the agency replies with "But they don't exist" (which they obviously will), the obvious line of questioning goes thusly:
1) "Do you have any documented study establishing your claim of non-existence?"
2) "How can you state non-existence when the plaintiffs have all this documented evidence of both existence and low population, both based upon commonly accepted wildlife management principles and evidentiary rules of the court, and you have brought nothing but opinion."
3) "How can you refuse the demand of remedy, which is that you study and document a status of endangered or not, in accordance with the ESA, since you have admitted right here that you have conducted no study on the species whatsoever, even to establish your claim of non-existence, which is itself an admittance of endangerment if they ever even existed in the past?"