If the earth gravity fluctuated between +50% and -50% over the course of 200 years, what specific adaptations would be useful to account for the change?
If gravity varied only between 0.5 g and 1.5 g over hundreds of years, most animals could adapt through changes to bone density as they grow up. Higher gravity might shorten the lifespan of some animals at the extreme (giraffes?) due to elevated blood pressure, but the major phenotypic changes are to bones and muscles.
Thankfully, mouse centrifuges are a thing (unfortunately there aren't any videos) so there are some studies in the 'hypergravity' area. Kwano N, et al. (2016) found that mice raised at 3 g have higher bone density and muscle mass, but they noticed that the inner ear was needed for some effects to occur. In their graphs, they have a "Sham" control group that was subjected to 1 and 3 g, but the "VL" (vestibular lesion) group had their middle and inner ear effectively destroyed by a labyrinthectomy, which is a (drastic, last-resort) procedure to treat vertigo, or here to see what the absence of the organic 6-axis IMU does.
So, animals on this variable gravity world would be slightly more adaptable in the bone and muscle-mass area and have slightly better inner-ear function. Higher changes over less time would demand more adaptability. The muscle mass might also include stronger blood vessels (arteries are wrapped in smooth muscle) to tolerate higher blood pressure.