Yeah, there are some really old responsive yoyo tricks where you throw the yoyo sideways, so it would be practically impossible to trace its actual invention to one person.
Hiroyuki Suzuki was one of the first to push horizontal stuff in 1A freestyles back in the early 2000s with his horizontal Eli hops and a few other tricks, and Shinya Kido was probably the first to really push it to the next level and start building significant portions of his freestyle out of longer or more complicated horizontal tricks. Those two are probably the most significant in terms of kickstarting modern horizontal 1A play (especially Shinya), but there were also a lot of yoyoers around that time who started contributing new horizontal stuff pretty quickly to build it up to what it is today.
Diabolo players were also a source of inspiration early on since there are a lot of horizontal diabolo tricks. Some people call horizontal play “Excalibur” style since that’s what it’s called for diabolos, so when Mickey started doing horizontal stuff they just borrowed the diabolo term.