'Pregnant Man' Thomas Beatie -- Busted for Stalking (UPDATE)

The famous "pregnant man" Thomas Beatie was arrested Thursday for felony stalking -- according to cops, he's engaged in some major spy games to track an ex over the last couple years.

Beatie was taken into custody by Phoenix police officers around 11:30 AM. He's currently being held in Maricopa County Jail without bail on one charge of stalking an ex -- at this point, it's unclear if it's an ex-girlfriend or his estranged wife.

Cops say the alleged victim found a GPS device hidden in her car ... and told Phoenix PD she suspected Beatie put it there. When detectives contacted him, they say Beatie admitted putting the device there 2 years ago ... and even said he'd replaced the battery in the unit several times.

Earlier this year, Thomas accused his estranged wife, Nancy, of verbally assaulting him and his new girlfriend ... and a judge issued a restraining order.

We called Nancy ... and she would not confirm or deny she's the woman Beatie allegedly tagged with the GPS.