Ah, I recognize the walker (and have a couple myself):
It's Sunyak, leader of the Maladroids, the main bad guys in Select's Convertors
Most Maladroids were Macross molds, but he's the Nikick from Orguss, the Big West Anime between Macross and Southern Cross (which I know nothing else about, except that it never got crammed into Robotech). The gerwalk is an official mode for him, and his card art depicts the pelvis as becoming a beak(!?), but he also has a flight mode which strikes me as similar enough to Straxus's cannon mode that I'd kitbash a bigger version if I could find one cheap.
He came with a rectangular, 3-barreled gun, but the shoulder wings are missing often enough from loose samples that yours looks to be in pretty good shape.
As for those other parts...I want to say the treads look like they could be from the G-Fighter(?) for Mobile Suit Gundam from Bandai's 2001 line (it splits up into a big mess of rubbery parts to combine with the Gundam in different configurations), but it could all just as easily be from Shogun Warriors or a different-named Diaclone import for all I know.