TECMO (previously known as Tehkan) is the Japanese company that develops and publishes games within the Monster Rancher series. They were founded in 1967.
Though recently UFO Interactive has been publishing the Monster Rancher games in English, they have recently merged with Koei, forming Tecmo-Koei. Cing has also been known to assist in the development of Monster Rancher games.
In the anime, the company is referenced by a company called MECTO.