Heroin was really popular for a long time. Seems like almost everybody that was big in music in the 40s, 50s or 60s got hooked at some point. It’s definitely a nasty drug, of course once you get hooked anything is nasty.
From what I've seen on the internet, meth is worse. I saw somewhere that the probability of getting clean once you've tried meth is less than six percent. One dose and you're hooked.
As far as musicians and horse, here's a partial list:
Kurt Cobain
Ray Charles
Charlie Parker
John Coltrane
Art Blakey
Gene Ammons
Miles Davis
Stan Getz
Dexter Gordon
Billie Holiday
J. J. Johnson
Hank Mobley
Gerry Mulligan
Anita O'Day
Art Pepper
Sonny Rollins
Zoot Sims
Sonny Stitt
I guess that's one thing the baby boom rockers had over the jazz guys. They stayed with non-fatal drugs mostly, although there are certainly the famous cases (Janice, Hendrix, Jim Morrison, Jerry Garcia, John Bonham, Keith Moon... gee, I guess that list mounts up too).