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.
4/8/2016 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.
4/8/2016 If you copy a double-faced creature, Altered Ego will be a copy of the face that’s up when Altered Ego enters the battlefield. Because Altered Ego is not a double-faced card, it won’t be able to transform. If an effect instructs you to return it to the battlefield transformed when it leaves the battlefield, it won’t return and will remain in its new zone.
4/8/2016 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.
4/8/2016 If the chosen creature is copying something else (for example, if the chosen creature is another Altered Ego), then your Altered Ego enters the battlefield as whatever the chosen creature copied.
4/8/2016 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.
4/8/2016 Any enters-the-battlefield abilities of the copied creature will trigger when Altered Ego enters the battlefield. Any “as [this creature] enters the battlefield” or “[this creature] enters the battlefield with” abilities of the chosen creature will also work.
4/8/2016 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.
4/8/2016 You can choose not to copy anything. In that case, Altered Ego enters the battlefield as a 0/0 creature, and is probably put into the graveyard immediately. It won’t have +1/+1 counters placed on it by its ability.
4/8/2016 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.