Loyalty costs on enchanted creature treat putting or removing loyalty counters on/from it to changing its toughness instead. A [+1] cost on a loyalty ability, for example, effectively has a cost of “increase enchanted creature’s toughness by 1,” though it still counts as a loyalty ability. Similarly, a [−X] cost becomes “Reduce enchanted creature’s toughness by X.” You can’t pay a cost that would reduce enchanted creature’s toughness to less than 0.