Brokkos, Apex of Forever
Legendary Creature — Nightmare Beast Elemental
Mutate 
(If you cast this spell for its mutate cost, put it over or under target non-Human creature you own. They mutate into the creature on top plus all abilities from under it.)
Trample
You may cast this card from your graveyard using its mutate ability.
Filip Burburan
Rulings
If you cast Brokkos with the permission granted by its last ability, you must pay its mutate cost to cast it. You can't choose to pay its mana cost or another alternative cost.
(wotc • 2020-04-17)
If another effect gives you permission to cast Brokkos from your graveyard, you may cast it for any cost associated with that permission.
(wotc • 2020-04-17)
Casting Brokkos with the permission granted by its last ability doesn't change when you can cast it.
(wotc • 2020-04-17)
Rulings
If you cast Brokkos with the permission granted by its last ability, you must pay its mutate cost to cast it. You can't choose to pay its mana cost or another alternative cost.
(wotc • 2020-04-17)
If another effect gives you permission to cast Brokkos from your graveyard, you may cast it for any cost associated with that permission.
(wotc • 2020-04-17)
Casting Brokkos with the permission granted by its last ability doesn't change when you can cast it.
(wotc • 2020-04-17)