Vastwood Animist — ruling 1

A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its {T} abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.

Vastwood Animist — ruling 2

When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.

Vastwood Animist — ruling 3

The value of X is determined as Vastwood Animist's activated ability resolves. The power and toughness of the Elemental won't change if the number of Allies you control changes later in the turn.

Vastwood Animist — ruling 4

If you control no Allies when the ability resolves, the land becomes a 0/0 creature and is put into its owner's graveyard as a state-based action. However, since Vastwood Animist is itself an Ally, the land will usually be at least a 1/1.

Vastwood Animist : règles, fonctionnement et interactions (MTG)

Vastwood Animist is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

: Target land you control becomes an X/X Elemental creature until end of turn, where X is the number of Allies you control. It's still a land.

How it works : Vastwood Animist

Original English text — Wizards of the Coast / Scryfall

#1March 1, 2010· Wizards of the Coast
A land that becomes a creature may be affected by "summoning sickness." You can't attack with it or use any of its abilities (including its mana abilities) unless it began your most recent turn on the battlefield under your control. Note that summoning sickness cares about when that permanent came under your control, not when it became a creature.
No other card is named in this ruling.
#2March 1, 2010· Wizards of the Coast
When a land becomes a creature, that doesn't count as having a creature enter. The permanent was already on the battlefield; it only changed its types. Abilities that trigger whenever a creature enters won't trigger.
No other card is named in this ruling.
#3March 1, 2010· Wizards of the Coast
The value of X is determined as Vastwood Animist's activated ability resolves. The power and toughness of the Elemental won't change if the number of Allies you control changes later in the turn.
No other card is named in this ruling.
#4March 1, 2010· Wizards of the Coast
If you control no Allies when the ability resolves, the land becomes a 0/0 creature and is put into its owner's graveyard as a state-based action. However, since Vastwood Animist is itself an Ally, the land will usually be at least a 1/1.
No other card is named in this ruling.