Horizontal scrolling shortcut in Linux

For a lot of things like this, xmodmap is useful, but I don't think it can remap keyboard+mouse combinations. However, you might have some luck using imwheel. It's actually fairly powerful - you can customize to have different mappings in different windows! For example, here's the first section of the sample file included in the source tarball:

"^mutt.*"
None,           Up,     Up
None,           Down,   Down
Control_L,      Up,     Page_Up
Control_L,      Down,   Page_Down

This will apply to mutt (or rather, windows with titles matching that pattern). The first field is keyboard modifiers for the input event, the second is mouse button/wheel input event, and the third column is output events. So, the first one maps wheel up to keyboard up, and the third maps left control+wheel up to page up.