Giant Slug — ruling 1

If the Slug changes controllers after the mana is spent, the player who activates this ability selects a landwalk during their next upkeep even if they don't control it at the time.

Giant Slug — ruling 2

When the ability resolves, it creates another ability that triggers at the beginning of your next upkeep. When that ability resolves, you choose a basic land type.

Giant Slug : règles, fonctionnement et interactions (MTG)

Giant Slug 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

: At the beginning of your next upkeep, choose a basic land type. This creature gains landwalk of the chosen type until the end of that turn. (It can't be blocked as long as defending player controls a land of that type.)

How it works : Giant Slug

Original English text — Wizards of the Coast / Scryfall

#1October 4, 2004· Wizards of the Coast
If the Slug changes controllers after the mana is spent, the player who activates this ability selects a landwalk during their next upkeep even if they don't control it at the time.
No other card is named in this ruling.
#2October 4, 2004· Wizards of the Coast
When the ability resolves, it creates another ability that triggers at the beginning of your next upkeep. When that ability resolves, you choose a basic land type.
No other card is named in this ruling.