Before the release of Homeworld, director Alex Garden had this to say about the third NPC "pirate" (or P3) race:

"P3, no one knows nothing about … they’re this extremely mysterious extremely powerful race that you’re almost unable to defeat and they’re completely evil. They’re like the great evil of the universe; they have no motivation that you can discern, they just come in and basically kick your ass, and do whatever the hell they want to and then they leave. And you’re left baffled and not understanding why this is happening. But you hate them, you hate them… You really hate them and there’s nothing you can do because they’re so much more powerful that you. But at some point in the game you might have a chance to get your revenge, if you’re lucky."

A starfish-like design sketch was released in some of the previews, and it was later revealed that the name of the P3 race was "T-Mat." Unfortunately, word got out before release that this starfish design would not be in the game.

"The star-shaped beast was the concept design for the T-Mat pirate mothership, but technical limitations forced us to axe it. A pity, it would have been fab." -Rob Cunningham, Art Director

The technical limitations apparently involved the large spaces between the starfish arms and a conflict with the collision-detection system. A replacement for the star-shaped ship was designed; it was more organic looking, and had stalactite-like spires that detached as separate frigates. Unfortunately, this version was also not used, and there are no T-Mat missions in the released game.

However, the T-Mat did make it into Homeworld after a fashion. The revised design for the T-Mat Megaship was used as one of the Galactic Council Member ships hyperspacing into Hiigara space with the Bentusi after the death of the Taiidan Emperor. (Actually, all of the the hyperspace signatures you see except for the Bentusi's are Megaships, but only one materializes before the scene fades to black.)

There are also entries in the .big file for a T-Mat Frigate and Destroyer; although there are no models for them, we have the stats. Both are similar to the Megaship; they have ludicrously high hit points and a single continuous-beam Ion Cannon.