Cerberus Watchdog is an autonomous threat detection framework designed to monitor the U2Ublockchain mempool in real time, identify potentially malicious transactions using artificial intelligencetechniques, and immutably record validated incidents on-chain through a dedicated smart contract. Bycombining mempool-level surveillance, AI-assisted classification, and blockchain-based reputationrecording, Cerberus Watchdog provides early-warning capabilities, transparency, and accountabilitywithin decentralized ecosystems. This paper presents the system architecture, design rationale,implementation details, detection workflow, preliminary evaluation, security and ethical considerations,limitations, and future research directions. The results indicate that AI-assisted mempool monitoring isa feasible and promising approach for proactive blockchain security.