Pena will be joining the AAA Round Rock Express, and will presumably be their everyday first baseman.
Pena, 36, was originally a first round draft pick of the Rangers in 1998, before being dealt to Oakland in the trade that brought Mario Ramos, Jason Hart, Ryan Ludwick and Gerald Laird to Texas.
Pena led the A.L. in homers for the Rays in 2009, but has not been very productive since then. He put up a .197/.330/.354 line for Tampa in 2012, and a .209/.324/.350 line for the Astros in 2013. He also had three plate appearances for the Royals in 2013, striking out all three times.
Pena most likely isn't a major league caliber player anymore, but given the lack of first base options the Rangers have, signing Pena and having him in AAA is probably worthwhile.