A card from BCOR that actually never came up in the Theme Guides is the Vatuka. This living pile of dirt is quite unusual among the biomechanical creatures of the Bionicle universe, but not actually unique. Other examples include the Avohkah who once lived in Karda Nui, the Fire Entity that Vakama fought at one point in the books, or on the less canon side the Snowbird from Quest for the Toa and the various bosses from Legend of Mata Nui.
The idea I came up with is to implement these types of creatures as Trap Monsters that support a certain Attribute, with the added gimmick that they can be activated/summoned from the hand while you control a Field Spell, because they are born from natural elemental forces (or something like that). So far the Vatuka is the only one, but if/when I get to any of the others, I plan to use the same style.
Interestingly, The Island’s Dark Tyrant also has the clause where you can activate it from your hand if you control a Field Spell. This is because the card shows Makuta controlling the aforementioned “natural elemental forces”, so it’s basically a pseudo-elemental. But only pseudo, because rather than a Trap Monster it only summons a Token. And because it’s a Makuta card, it also works from the GY.
The Element Lords from Spherus Magna are also technically elementals, though of a significantly different nature. Maybe I’ll work them into this concept somehow.