When I was a kid Power Rangers weren't a thing yet (that was my teen years), but we had the same problem with Voltron and the only robeasts being tiny 3 3/4" figures. But when I was a kid we had these:
Inhumanoids monsters were massive at the time and a lot of fun to use for fighting big robots with. I still have Tendril (the green one) somewhere in storage and he is such a great figure. And hey, he has light piping if that's your thing.
Found a pic of him with some common smaller figures for scale.
Unfortunately the aftermarket price on these are pretty crazy. It's too bad that one of those companies that make the big hard plastic dinosaurs and sometimes dragons you find at Walmart in the car aisle (for some unexplained reason) doesn't make big generic monsters to fill that need.
Emerje