These are two versions of the Romulan Bird of Prey. The gray one was designed and used in the Classic Star Trek Series. The green one was used in an episode of Enterprise. It is supposed to be the same ship, but artistic license was used considering our advancement in special effects. They did a good job. Though dramatically different in many respects, it still looks like the classic ship. The same way they handled the Enterprise improvements in the films over the TV series.