Zygon Infiltrator

Body-print — {2}{U}: Tap another target creature and put a stun counter on it. Zygon Infiltrator becomes a copy of that creature for as long as that creature remains tapped. Activate only as a sorcery. (If a permanent with a stun counter would become untapped, remove one from it instead.)

Doctor Who

If another creature becomes a copy of Zygon Infiltrator, it will become a copy of whatever Zygon Infiltrator is currently copying (if anything).

If it becomes a copy of a token creature, it copies the original characteristics of that token as defined by the effect that created the token. It won't become a token creature.

When Zygon Infiltrator becomes a copy of a creature, it's neither entering nor leaving the battlefield. Any enters-the-battlefield or leaves-the-battlefield abilities won't trigger.

Zygon Infiltrator copies the printed values of the creature plus any copy effects that have been applied to it. It won't copy any other effects that have changed that creature's power, toughness, color, and so on. Zygon Infiltrator won't copy any counters on the creature, but it will retain any counters it already had on it.