Leyline of Transformation

If this card is in your opening hand, you may begin the game with it on the battlefield.
As this enchantment enters, choose a creature type.
Creatures you control are the chosen type in addition to their other types. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.

Duskmourn: House of Horror

If Leyline of Transformation is in your opening hand, you may begin the game with it on the battlefield.
As Leyline of Transformation enters, choose a creature type.
Creatures you control are the chosen type in addition to their other types. The same is true for creature spells you control and creature cards you own that aren't on the battlefield.

Duskmourn: House of Horror Promos

A player's "opening hand" is the hand of cards the player has after all players have taken mulligans. If players have any cards in hand that allow actions to be taken with them from a player's opening hand, the starting player takes all such actions first in any order, followed by each other player in turn order. Then the first turn begins.

Replacement effects that modify creatures of a certain type as they enter will apply after you apply this effect. For example, if Warrior is the chosen creature type and you control Bramblewood Paragon (a creature with "Each other Warrior creature you control enters with an additional +1/+1 counter on it."), a Runeclaw Bear would enter with an additional +1/+1 counter.

To choose a creature type, you must choose an existing creature type, such as Human or Survivor. You can't choose multiple creature types, such as "Human Survivor." Card types such as artifact can't be chosen, nor can subtypes that aren't creature types, such as Kaito, Vehicle, or Treasure.