Thexder is a Turrican-like (probably inspired Turrican).
Thus also a Metroid-like (predating it by 1 year).
First released for NEC PC 8801 in Japan in 1985 (!), it is by the same developers as Zeliard which is also in the suggestions https://www.dosgameclub.com/forums/topic/zeliard-1990/
Sierra published the DOS version in 1987.
It therefore is a sidescroller predating Commander Keen.
It is also interesting that it inspired Amiga fav Turrican.
It’s not so interesting to have a side scroller that predates Keen. The interesting thing about Keen was *smooth* scrolling. Thexder scrolls tile-by-tile at what looks like 10fps which gives the game more than enough time to update the entire screen.