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 ${\mathrm{Eu}}_2{\mathrm{CdAs}}_2$. The compound undergoes antiferromagnetic ordering at $T_{\mathrm{N}}=9\mathrm{K}$ and exhibits semiconducting behavior with a low-temperature activation energy of 3.2 meV, achieving a large CMR effect of $-{10}^4\%$ at 9 T. Below $T_{\mathrm{N}}$, 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 band-gap engineering serves as an effective strategy for tailoring magnetotransport, establishing Eu-containing Zintl phases as a versatile platform for exploring and manipulating CMR effects.

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• Antiferromagnetism
• Colossal magnetoresistance
• Magnetic semiconductors