Hello anon 🙂 Her supporter is a deer, a white hind. The press release at the time said: “The Supporter assigned to The Duchess of Cambridge is a white hind, which has had continuing Royal connections in England since the 14th Century… The white hind has had a long tradition of connections to the Royal Family in England since the fourteenth century. It was the Badge of Joan of Kent (c. 1328-1385), Princess of Wales, better known as the Fair Maid of Kent. In 1467 an inventory of ornaments and relics at Westminster Abbey included a red altar-cloth and frontal with gold lions and white hinds for the altar of King Henry V. In 1529 King Henry VIII had three hinds carved amongst the eighteen beasts at Hampton Court. The white hind contrasts with the white hart, the favourite Badge of the Fair Maid of Kent’s son Richard II. The hart or stag is the male red deer and the hind the female. A hind is a forest dweller and stands well next to a Shield where acorns are the principal feature.”
So it was chosen to emphasise her Britishness, her position as a future Princess of Wales, and her love of the countryside and nature.