A Minelayer is a large Ork vessel used to provide fire support and place mines high above enemy positions. The mine's repulsor fields enable it to stay hovering in the air. Once an enemy attempts to cross the ground underneath a mine, they will collapse the delicate repulsor field and the mine will plummet downwards towards the target area.
It was seen in the Chapter 2 where it leaves a Slugga Nob and several mobs of Ork Boys and Gretchins.
The next appearance is in Chapter 4 where Titus must defeat it using Plasma Cannon.
The 3rd appearance is in Chapter 9 where Titus must defeat it with Heavy Bolter.