Creature — Shapeshifter
This spell can't be countered. You may have Altered Ego enter the battlefield as a copy of any creature on the battlefield, except it enters with X additional +1/+1 counters on it.
X can be 0. Altered Ego won’t enter with any additional +1/+1 counters, and it will just be a copy of the chosen creature.
If the chosen creature is a token, Altered Ego copies the original characteristics of that token as stated by the effect that put the token onto the battlefield. Altered Ego isn’t a token.
If the chosen creature has in its mana cost, that X is considered to be 0. The value of X in Altered Ego’s last ability will be whatever value was chosen for X while casting Altered Ego.
If Altered Ego somehow enters the battlefield at the same time as another creature, Altered Ego can’t become a copy of that creature. You may choose only a creature that’s already on the battlefield.
Altered Ego copies exactly what was printed on the original creature (unless that creature is copying something else or is a token; see below). It doesn’t copy whether that creature is tapped or untapped, whether it has any counters on it or any Auras and Equipment attached to it, or any non-copy effects that have changed its power, toughness, types, color, or so on.