Discovering antiferromagnetic materials with a sizable colossal magnetoresistance (CMR) effect and precisely tuning their responses remain significant challenges in materials science. This work reports the synthesis, phase transitions, and magnetotransport properties of the narrow-gap Zintl arsenide Eu₂CdAs₂. The compound undergoes antiferromagnetic ordering at TN = 9 K and exhibits semiconducting behavior with a low-temperature activation energy of 3.2 meV, achieving a large CMR effect of -10⁴% at 9 T. Below TN, a distinct butterfly-shaped magnetoresistance hysteresis emerges without any macroscopic ferromagnetic order, revealing a transport anomaly likely dictated by AFM domain dynamics and/or the formation and percolation of localized magnetic polarons. Furthermore, a systematic survey of reported Eu-based Zintl phases uncovers a nearly exponential scaling trend between the maximum CMR magnitude and the low-temperature activation energy. This correlation demonstrates that bandgap engineering serves as an effective strategy for tailoring magnetotransport, establishing Eu-containing Zintl phases as a versatile platform for exploring and manipulating CMR effects.