Version experimental 5bb83dc (0.c build 3862)
Occasionally, a large horde will spawn, with just one type of zombie. Here is an image with just corrosive zombies spawning. Once I had a horde of about 30 or so feral predators spawn. Another time it was a few dozen zombie masters. Luckily I haven’t come across this situation with zombie necromancers.
This occurs without the intervention of zombie masters nearby to upgrade standard zombies.
I tried attaching an image with the situation, but it looks like the upload folder is full.