2015-11-04 : The value of X is calculated as Pathbreaker Ibex’s ability resolves. Notably, if you attack with two Pathbreaker Ibex (and control no other creatures), the first ability to resolve will give those Ibex +3/+3. The second ability will then give them +6/+6.

2017-07-14 : If this creature’s power is negative as its ability resolves, X is considered to be 0. (This is a change from previous rulings.)