Keruga, the Macrosage
Legendary Creature — Dinosaur Hippo
Companion — Your starting deck contains only cards with converted mana cost 3 or greater and land cards. (If this card is your chosen companion, you may cast it once from outside the game.)
When Keruga, the Macrosage enters the battlefield, draw a card for each other permanent you control with converted mana cost 3 or greater.
Illustrated by
Ikoria: Lair of Behemoths Promos#225sRareEnglishLegality
Rulings
If a card in a player’s deck or a permanent on the battlefield has 
in its mana cost, X is considered to be 0.
The converted mana cost of a token that isn’t a copy of another object is 0. A token that is a copy of another object has the same mana cost as that object.
Your companion begins the game outside the game. In tournament play, this means your sideboard. In casual play, it’s simply a card you own that’s not in your starting deck.
Before shuffling your deck to become your library, you may reveal one card from outside the game to be your companion if your starting deck meets the requirements of the companion ability. You can’t reveal more than one. It remains revealed outside the game as the game begins.
The requirements of the companion ability apply only to your starting deck. They do not apply to your sideboard.
If more than one player wishes to reveal a companion, the starting player does so first, and players proceed in turn order. Once a player has chosen not to reveal a companion, that player can’t change their mind.
If you reveal a companion outside the game, for as long as it remains there, you may cast it any time it’s legal to cast. Once you do, it comes into the game and behaves like any other card you’ve brought into the game. For example, if it’s countered or destroyed, it’s put into your graveyard, remaining in the game.
The companion’s other abilities apply only if the creature is on the battlefield. They have no effect while the companion is outside the game.
The companion ability has no effect if the card is in your starting deck and creates no restriction on putting a card with a companion ability into your starting deck. For example, Zirda may be in your starting deck even if your other permanent cards don’t all have activated abilities.
You may have one companion in the Commander variant. Your deck, including your commander, must meet its companion requirement. Your companion is not one of your one hundred cards.