A simple plugin to edit spawn rate probabilities. It's got a big config file with every mob in it. Set the probability to whatever you want, but 1.0 is the maximum.
1.0 - The mob spawns like normal
0.0 - The mob won't spawn
The plugin wont crash or anything if you try any higher than that (except if you exceed the value limit for the type double), but it won't have any effect, the mobs will just spawn like regular.

It's also possible to modify the spawn rate of the dimensions in general, this is done using the world's name suffix (like "_nether"), so if you change it this wont work.

This will not affect entities not listed in the config such as arrows, eggs or the Ender Dragon.

Disclaimer: This plugin was requested on reddit and has only been tested on paper. Since paper is built on spigot and I haven't used any complicated code, this will probably work on spigot and bukkit as well though. If not, please message me.