Added in 1.25.3, the patch notes claim that portal Boss Cards are "the most powerful cards in the game."
How to ObtainEdit
Defeating a boss 15 times, clearing Onyx difficulty, unlocks the Boss Card. Wins after unlocking give a chance to win, similar to any other tier. Each card is the avatar of the boss with Gold rarity. Portal boss cards are available with Diamond Spins during their respective Events.
Probabilities (Quoted from Chinzilla)Edit
We can definitely give you a range so you have a better idea of the drop rate chances for the bosses. Since most of the screens in the game are pretty packed, we couldn’t find a clean and uncluttered way to display all the percentage drop rates. To give you an idea, Chloe (first boss) has a 1.5% chance to drop a rare, while Clyde (last boss) has a 6% chance for a rare card. But you also need to unlock the rare cards from the prize pool to have a chance to get a rare drop. The percentages for uncommon and common are higher than rare. Overall for card drop rates in general, Chloe will have a 28% chance that you will get at least a card, while Clyde has a 42% chance that you will at least get a card.
All the rates are between those two percentages, hopefully giving you the min and max will help give you a better idea. :)