On a guitar, how do you make it sound like a horse whinneying? I know you use the whammy bar, but I don't know what exactly to do.

If you have never heard this, a good example of it is in the song Nemesis by Arch Enemy.
play around with them all, go mad, have fun. guitar isnt supposed to be a research subject!
i think you're actually supposed to make a pinch harmonic, push the bar down, and let it go, so it vibrates up and down.
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I'm not sure if this is right, but I thought it was pinch harmonic, push down on the whammy bar to heighten the note, then you release it and push the other way slowly to lower the note.
... Pushing down on the bar lowers the pitch, unless you're doing it the Dime way and have the whammy bar facing the rear of the guitar.
I thought for Dimebags sound it was a natural harmonic and lift the bar up (or have the bar reversed and push it down)

Alternatively you could pinch harmonic it push the bar down then raise it up
Dimebag woud flick the string he wanted then dive down on the bar, then get the natural harmonic, then pull up and go a bit wild. It's really easy with practice.
yea theres videos on youtube illustrating how dimebag does it. bad horsie is a great example too. just have fun with your whammy.
If you want to have control over the squeal, you use you whammy bar. Find a good natural harmonic on one of the last 3 strings and as soon as you hear that squeal go wild on the bar. Thats how I do it, but there are probably others who cant do and explain it a lot better.