Ink cores (インクコア Inku koa) are rare generic stage enemies that are found only in Evanescent Existence.
Ink cores are, as the name suggests, presumably made of ink. The names of these enemies were unknown for years until they were revealed to return in Book of the Cafe. "Large ink core" (ラージインクコア) was the name of the one appeared in the first game, and it's presumed there may be smaller variants of these enemies. JynX mentioned in an interview that the ink bird was intended to be one of Tsurubami Senri's shikigami, which could give the assumption that all over ink cores are, as well.
An ink bird, unused in EE
A singular large ink core appared on stage 2 of Evanescent Existence, then again with two presented on stage 3, intended to be Tsurubami's shikigami. An unused version, an ink bird (インクバード), was left in the data files, and was scrapped from the game due to them being shikigami was not obvious.
They did not make a return from Earthen Miraculous Sword onwards.
Book of the Cafe
Ink cores, both large ink cores and ink birds, are set to return in Book of the Cafe, being revealed in the Len'en 7th Anniversary Video. Exactly how they'll function remains to be seen. Notably, Evanescent Existence is listed in the ink bird's "Appearances" section.