There are currently only two known Sentries on testbed, these of which have never been seen and probably will never be seen in the normal game. The Varp and the Gunner Variant of the Flash Sentry.
Thought-to-be Scrapped or Removed Sentries.
The Varp was a large, Collider Lite blue, irregular Decagon. It had a large drone spawner in the back of the body. It was only accessible by clicking the "Sentries" button in Developer Events. There are two reasonable and possible reasons it was never added into the regular game, and the third is a mere theory. For the reasonable and possible reasons, 1. It was bigger than a Real-Size Mothership, thus it would turn out as a better boss. Even then, for reason number 2, The Varp made HUNDREDS of devastating drones, so it may have been too powerful for even being a boss. However, a theory from Purrfect Speed says that maybe it was originally going to be in the game, but it got forgotten. He also believes maybe it was originally in the game but it got removed due to reason number one and reason number two. Either way, it may never be known why the Varp is currently not in woomy.arras.io. Edit: It was re-added into the game.
Flash Sentry Gunner Variant
The Flash Sentry's Gunner Variant appeared as a Flash Sentry, but with two pellet guns on each side of the back. As far as color and shape however, it was a regular Flash Sentry. Unlike the Varp, it wasn't too overpowered, and it was the same size as an average Flash Sentry, however it never made it out of Developer Events. It is still unknown however if it never made it out, or if it is actually just extremely rare, like it's "cousin." Flash Sentry's Gunner Variant only being accessible in Developer Events is actually just another theory by Purrfect Speed.
Beta to be Added or to be Scrapped Sentries
The Swarming Square and AWP-39 Lite were seen in a recent developer event.