Jegantha, the Wellspring

Mana cost: {4}{R/G}
Type: Legendary Creature — Elemental Elk
Power / toughness: 5 / 5
Set: Multiverse Legends #109 (rare)

Companion — No card in your starting deck has more than one of the same mana symbol in its mana cost. (If this card is your chosen companion, you may put it into your hand from outside the game for {3} as a sorcery.)
{T}: Add {W}{U}{B}{R}{G}. This mana can't be spent to pay generic mana costs.

Jegantha, the Wellspring has 10 printings, first in Ikoria: Lair of Behemoths (2020). EDHREC ranks it #3,813 among Commander cards. It appears most in Ashling, the Limitless decks (13,211 on EDHREC). 1 public deck on TCG runs it. It has 5 official rulings.

Legality

commander: legal
legacy: legal
vintage: legal
pauper: not legal
modern: banned
pioneer: banned
standard: not legal
brawl: legal

Public decks that run Jegantha, the Wellspring

Dance of the Elements - Lorwyn Eclipsed Commander Deck, led by Ashling, the Limitless

Commanders that run Jegantha, the Wellspring most

Deck counts from EDHREC.

Ashling, the Limitless: 13,211 decks
Sisay, Weatherlight Captain: 5,724 decks
Jodah, Archmage Eternal: 3,573 decks
Horde of Notions: 1,802 decks
Progenitus: 1,170 decks
Mass of Mysteries: 1,028 decks
Jenson Carthalion, Druid Exile: 898 decks
Niv-Mizzet Reborn: 809 decks

Often played with Jegantha, the Wellspring

Cards that show up in the same decks on EDHREC.

Rulings

2020-04-17: 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.
2020-04-17: 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}.
2020-06-01: 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
2020-06-01: Once you put your companion into your hand, it 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.
2023-04-14: You can spend mana from Jegantha's mana ability to pay for a hybrid symbol such as {2/W}, but only if you choose to pay the colored mana component, not the generic mana component.