I've been to couple places.
I don't really socialize with the metal crowd tho.
They could've meant Strahov 007 (saw Manilla Road there), Nová Chmelnice (saw Carnivore there), Modrá Opice (they do grind things) etc., I saw GWAR in Acropolis, but that's a bit more mainstream place, there's a new place in my neighborhood where Impertuous Ritual are supposed to play in couple months time, I want to see what's up.
Dude if you managed to come across horses in Prague then you've definitely done all the touristy things.
Prague 9 is not exactly my area of expertise, I grew up in Prague 4, now I live in Prague 5, worked in Prague 6, but the centre of the city is where my heart is.