The Rocket Triangle (RT) is a polygon that has a 10% chance to spawn in the place of a Triangle. It is a Triangle that can fight back.
RTs normally (when in idle) looks like a regular Triangle, but slightly bigger. However, when a Player enters its detection zone (a circle centered on the RT w/ a radius of about 10 map squares), it will sprout a shorter Machine Gun Cannon on one of its sides, and will start to attack the player.
TechnicalThe RT's Trapezoidal Cannon is quite similar to a Machine Gun's Cannon, as they have the exact same Bullet Accuracy/Spread and Bullet Penetration. The RT's cannon does less Bullet Damage, while having more Bullet Speed and Reload. Its extremely high Recoil propels it at a very high speed like a rocket, giving its name.
When a Player enters a RT's aforementioned detection zone, The RT will reveal its Cannon. Then It will aim the vertex of its body that's farthest away from the Cannon at the Player, and the cannon directly opposite the Player. It will be "locked" into that orientation. Then, its Cannon will start firing, propelling itself towards the Player. It will keep moving in that direction until is is destroyed (either by a Player or by dying from ramming things), or reaches the edge of the Map, where it destroys itself.
To avoid being ambushed by RTs, be of the lookout for Triangles that are larger than other ones. If one is within the proximity, avoid that area at low levels. If high-leveled, trying to destroy the RT is possible, though make sure to move immediately when the Cannon is revealed, as the RT cannot change its orientation once the Cannon is visible.