Creature — Horse Illusion
4/3
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→ Near-infinite death triggers, Near-infinite ETB, Near-infinite lifeloss
→ Infinite LTB, Infinite ETB, Infinite creature tokens
→ Infinite blinking of nonland permanents, Infinite creature tokens, Infinite ETB
→ Infinite turns, Lock
→ Infinite LTB, Infinite ETB, Infinite creature tokens
→ Infinite blinking of nonland permanents, Infinite creature tokens, Infinite ETB
Phantom Steed has received a nonfunctional update to its wording. The printed card refers to the “exiled creature.” The corrected text (“exiled card”) is included above.
If Phantom Steed leaves the battlefield before its ability resolves, the target creature won't be exiled.
Auras attached to the exiled creature will be put into their owners' graveyards. Any Equipment will become unattached and remain on the battlefield. Any counters on the exiled creature will cease to exist. When the card returns to the battlefield, it will be a new object with no connection to the card that was exiled.
If a token is exiled this way, it will cease to exist and won't return to the battlefield. Phantom Steed's last ability won't create any tokens.
Although the token created by the triggered ability is attacking, it was never declared as an attacking creature (for the purposes of abilities that trigger whenever a creature attacks, for example).
You declare which player or planeswalker the token is attacking as you put it onto the battlefield. It doesn't have to be the same player or planeswalker Phantom Steed is attacking.
If the exiled card has 
in its mana cost, X is 0.
Any enters-the-battlefield abilities of the copied creature will trigger when the token enters the battlefield. Any “As [this creature] enters the battlefield” or “[This creature] enters the battlefield with” abilities of the copied creature will also work.
If Phantom Steed somehow exiles a noncreature card, such as an animated land, it will still create a copy of that card when it attacks. It enters the battlefield tapped and will be sacrificed at the end of combat. However, it is not attacking and it is not an Illusion.