2013-04-15 : You don’t choose the value of X. The bonus Feral Animist gets is based on its power when the ability resolves. For example, if you activate the ability twice, the first ability will give it +2/+0 and the second one will give it +4/+0.

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.)