Tex. R. Civ. P. 81
As amended through August 27, 2024
When the defendant sets up a counter claim, the plaintiff may plead thereto under rules prescribed for pleadings of defensive matter by the defendant, so far as applicable. Whenever the defendant is required to plead any matter of defense under oath, the plaintiff shall be required to plead such matters under oath when relied on by him.
Tex. R. Civ. P. 81