Antimicrobial resistance (AMR) poses an escalating challenge to human and veterinarian spaces, yet little is known about its early development within companion animal microbiomes. This study explores the evolution of AMR genes (ARGs) in the gut microbiome of Labrador Retriever puppies across their first year of life. Using shotgun metagenomic sequencing, we profiled microbial community composition and ARG prevalence at 4, 7, and 12 months of age. Preliminary data revealed that puppies shared highly similar microbial profiles at 4 months, dominated by Segatella copri (63 ± 26%) and Megamonas uniformis (6 ± 3%). Despite this compositional homogeneity, the resistome exhibited wide diversity, averaging 12 ARGs per million bases sequenced, primarily β-lactamase, tetracycline, and macrolide resistance genes. Rarefaction analysis demonstrated that approximately 1 Gbp of sequence depth is necessary for full resistome characterisation, underscoring the hidden diversity of AMR within animal microbiomes. Notably, ARG abundance appeared uncoupled from major taxonomic shifts, suggesting environmental exposure may be key drivers. The low abundance of putative pathogens detected amongst the samples (e.g., Campylobacter, Helicobacter) indicates these genes are harboured largely by commensal taxa. These findings provide baseline data for understanding how AMR evolves during host development and highlight the potential for early-life establishment of resistance reservoirs in domestic dogs. Improved surveillance of animal microbiomes could enhance predictive models of resistance transmission between companion animals and humans.