Jegantha, the Wellspring

Jegantha's companion ability compares the exact symbols in the mana costs of cards in your deck. If any one card has the same symbol twice, such as {X}{X}{R} or {(r/g)}{(r/g)}, the companion condition isn't satisfied.

A generic mana cost is usually represented by numeric mana symbols ({1}, {2}, and so on) and also {X}. It is any cost requiring mana where that cost isn't {C}, {W}, {U}, {B}, {R}, or {G}.

Wizards of the Coast has issued functional errata for the Companion mechanic. Instead of casting companions from outside the game: Once per game, any time you could cast a sorcery (during your main phase when the stack is empty), you can pay {3} to put your companion from your sideboard into your hand. This is a special action, not an activated ability. It happens immediately and can’t be responded to. It can’t be countered or stopped by cards like Phyrexian Revoker. For more information please see https://magic.wizards.com/en/articles/archive/news/june-1-2020-banned-and-restricted-announcement