Use this command when close to a plain unicorn to get a surprise:
/changeHorse ~ 3840
Note: This only works in Minecraft 1.9, with the 1.4.18 version of the mod, and only on vanilla unicorns (it actually works on any magical horse with unicorn hide, but will remove other features you might like, such as the hippocamp tail).
The changeHorse command can be used to modify nearby horses easier than using the entitydata command. It's basically a better way to use cheat codes. You can find out more here.
P. S. -- I injured my arm and hand this past week, so development has stopped for the moment (typing is painful). I'll get back to it when I get better.