This happened twice so far during a Drill Pod run level 14+.

Heroes:Ken, Josh, Elise, and Rakya

Chimera Keepers, two which are stacked on one another spawns during a wave. They do not have an indicator, so when I kill all the indicated monsters I have to look for the Keepers in unpowered rooms. The most I got during one wave was 4 of these(4 rooms with 2 Keepers each)

*There is one normal Keeper and an Elite Keeper in one room.

This has happened with normal Keepers spawning in different rooms, though they have the room indicators and are not stacked.