Source (Streets of New Capenna - "Streets of New Capenna [...] had a three-color shard theme (a central color and its two allies), so it made a cycle of cards that had a mana cost with its central color and two different hybrid costs, each with the central color and one of the ally colors. This would allow you to play the card with mana of its central color or one or two of the ally colors.")
Source (Streets of New Capenna - "There was also a common cycle of monocolor creatures with a hybrid activation of the two ally colors called Initiates. This allowed us to make three-color commons that played into the shard theme without making cards that were hard to play. (As a normal rule, we tend to avoid making too many common traditional gold three-color cards.)")
Source (The Seer Cycle - "[...] They were moved to uncommon, and a new cycle of one-shot spells took their place at common. The common spells had the same scalable effect as their respective Seer.")
Source (Use of Multicolor - Power Spell Cycle (Rare) - "I believe this cycle turned into the Ascendancy enchantments, such as Jeskai Ascendancy ." - Empires (Rare) - "[...] We went through many iterations, but the one we went with is a cycle of enchantments that cost the wedge's colors. [...] It's possible this is the cycle that became the Ascendancy enchantments.")
Source 1 (Question #7 - "Invasion, made use of Unglued II's split cards") - Source 2 (#1) (tie) Split cards (3) - "Split cards were first created for Unglued 2, but eventually saw print in Invasion.")