。
Wild animals were roaming the grasslands.
Literal
Wild [gen-の] animals [subj-が] grassland [obj-を] were-walking-around.
歩き回る is a compound verb from 歩く ('walk') + 回る ('go around') — 'walk around, roam.' Compound verbs with ~回る often add a sense of wandering or sweeping movement. 草原 takes を (not で) because it's treated as a traversed path/area rather than a static location.