FF_HORIZONTALFLIP is a frame flag analogous to
FF_VERTICALFLIP that flips the sprite horizontally:
in combination with
mobj->mirrored inverts the angle used to calculate sprite rotation and flips the sprite accordingly. In effect, this allows asymmetric objects to be easily mirrored, creating left-handed Fang:
rollangle, does not behave the same way on models as it does on sprites: it acts exactly like
mobj->mirrored: flipping the model across the mobj's angle, rather than relative to the camera angle.