This paper explores AI-mediated human-pet musical co-creation from an interdisciplinary perspective, leveraging recent advancements in animal-assisted therapy. These advancements have shown significant psychosocial benefits, especially in reducing anxiety and enhancing social engagement. Building on these findings, this study innovatively employs pet vocal timbres as 'digital avatars' to enhance emotional investment during the music creation process. We propose PetCoCre, a novel system that applies pet vocal timbres in three distinct character paradigms within AI music creation: (1) PetRhythm: using pet voices as rhythmic percussion through beat synchronization. (2) PetMelody: enabling pet voices to act as melodic instruments via pitch-shifting alignment. (3) PetVocalia: utilizing pet vocal timbres as the target timbre for SVC (Singing Voice Conversion), where the converted singing voice replaces the original singer's voice, thus preserving the original semantic content. Beyond these character paradigms, our technical innovation lies in proposing SaMoye, the first open-source, high-quality zero-shot SVC model that effectively overcomes existing methods' zero-shot limitations by employing mixed speaker embeddings for timbre enhancement and leveraging a large-scale singing voice dataset. In our experiments, we collected dog and cat vocalization data from pet stores and conducted experiments with 30 participants. Results demonstrate that the human-pet co-creation mode led to significant enhancements in pleasure and creative satisfaction compared to solo AI music generation, along with a significant reduction in participants' anxiety levels. Through collaborative art creation, this research pioneers new paradigms for animal-assisted therapeutic interventions and expands the boundaries of AI-assisted creative collaboration.