[Jolene van Antwerp] “Could it be that FCP is seeing it as 25fps, which is why it keeps 100% speed – which is actually slow motion 120fps.”
That is exactly correct. The footage is “over cranked”.
[Jolene van Antwerp] “I would actually like to do a test and set the video frame rate the same as the shooting rate (video rate 120fps & shooting rate 120fps), to see if FCP will automatically conform the speed when dropped into a 25fps sequence. Do you think this might work? “
What would be the goal? I’m not sure if the camera will allow a project rate of 120fps, but even if it did, at some point you will have to place the footage in a 25fps timeline. So if you put 120fps footage in a 25fps timeline, FCPX will skip frames to play it back, so it is essentially the same thing as speeding up the 120fps @ 25 footage. When shooting 120 or slow motion, all of the motion characteristics of each frame will look different than if you would have shot at 25fps originally. Each frame will have less motion blur as there is less motion per frame, if that makes any sense. Each frame, essentially, is more “sharp” which is why it doesn’t look right to your eye when speeding it up. The motion looks somewhat unnatural. Another way to put it is that a bug flying in the sky will move a greater distance in one frame of 25fps footage than 120fps footage resulting in more motion blur. I would definitely encourage you to test because it’s fun! ☺ You can play with different shutter speeds to effect motion blur per frame. At some point, you will need to decide f you are shooting for slow motion or regular motion. It looks like you work in wildlife, so you probably want it all! ???? Which is completely understandable, by the way.
With your current footage, I would turn on optical flow and you should see a difference in the motion. To do this, you put the footage in the timeline, speed it up 480%, and then go to the retiming wheel and choose Video Quality > Optical Flow and let FCPX analyze the footage. Like this:

Jeremy