Let X be a p-divisible group over a regular scheme S such that the Newton polygon in each geometric point of S is the same. Then there is a p-divisible group isogenous to X which has a slope filtration.
Thomas Zink. "On the slope filtration." Duke Math. J. 109 (1) 79 - 95, 15 July 2001. https://doi.org/10.1215/S0012-7094-01-10913-7