In a world I've been creating, space and time are mutable. Not in the traditional sense, but because non-sentient magic crystals warp space and time as their means of reproduction.
Now, this warping can't affect sentient creatures or their artifice, allowing people to exist for more than 15 minutes without being wiped, and this is all well and good. By actual issue is that the crystals make monsters in the space they shape. All the time. They're always making new breeds. And this is also fine, but it leaves a dilemma.
How can I justify any given type of creature being created more than once?
There's no guiding intelligence behind this system of constant re-creation, and I'd rather not say "Because reasons."
As an added detail, anything created does have to abide by the laws of physics, though given the presence of magic these are obviously a bit more pliable than our own. Basically, no generation of Cthulu's daydreams.