The Norse gods have different ways of getting around: Odin and Heimdall have horses, Freyr has a boar, and Thor has two goats to pull his wagon. Freyja’s choice was a little more unusual: she had two cats to pull her chariot.
Where did the idea that Flidais rode in a chariot drawn by deer come from? It’s not in her main legend, the Táin Bó Flidais, nor in the follow-on story, the Táin Bó Cúailnge. It’s an attractive image, bringing to mind the Middle Eastern goddesses with their lion-drawn chariots, Freyja with her cats, and Nerthus in her wagon drawn by heifers.