Same error message form the 1.6.82-j launcher on my Linux Mint 17.3 machine this evening after everything was working this morning.
Tried using CrossOver to install the Windows version, with 1.6.82 launcher. Same error message from that one.
Java HotSpot(TM) 64-Bit Server VM warning: Using incremental CMS is deprecated and will likely be removed in a future release
Error: Could not find or load main class net.minecraft.client.main.Main
Can run 1.12.2 vanilla using MultiMC, so the problem must be with the 1.6.82-j launcher on Linux.
Yup. Something happened since last week. I had three rideable horses last week and none now.
10:15 UTC today.
Update to 1.6.84-j launcher fixes problem in Linux Mint 17.3. Vanilla and Optifine versions both run successfully.
Using Windows 1.6.84 launcher update through CrossOver runs Vanilla too. (Haven't tried Optifine with that one.)