For endohedral metallofullerenes (EMFs), that is, fullerenes encapsulating metallic species, cage size is known to be an important factor for cluster configuration adoption; however, the impact of the cage shape on the cluster geometry fitting remains poorly understood. Herein, for the first time, four dierbium-carbide EMFs with C90 cages, namely, Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90, were successfully synthesized and fully characterized using a combination of mass spectrometry, single-crystal X-ray diffractometry, vis-NIR, Raman and photoluminescence spectroscopies, and cyclic voltammetry. In particular, the fullerene cages of C2(43)-C90 and C2(44)-C90 are crystallographically identified for the first time. Interestingly, the ErEr distance of the major sites in Er2C2@C2(43)-C90, Er2C2@C2(40)-C90, Er2C2@C2(44)-C90, and Er2C2@C1(21)-C90 is 3.927, 4.058, 4.172, and 4.651 Å, respectively, which increases gradually with an increase in the major axis of the cage. Moreover, the bond length of the inner C2-unit decreases progressively with an increase in the ErEr distance, indicating that the inserted C2-unit can serve as a molecular spring to support the strong metal-cage interactions within cages with the same size but different shapes. Hence, the role of cage shape on the cluster configuration is unveiled safely for the as-obtained Er2C2@C90 isomers.