Veigues Tactical Gladiator is side scrolling and a shooter but not exactly the same as MetalSlug. You controll a giant mech shooting down weapons as you roll though stages at the end og each stage you get a grade depending on how you did you allows you to improve your armor, energy shielding and jump jet as well as your punch arm.
Shock Man is kinda like Megaman.
Atominc Robo Kid is more of a space shooter... sort of.
There just arnt many serious side scrollers that arnt space ship shooters for the TG16/PCE. Most ar top down like the Ikari Warriors style games Like Diehard and Bloody wolf.