In the novel Postern of Fate, Mathilde is a rocking horse left by previous owners at the Laurels. According the Isaac Bodlicott, the handyman/gardener who came to help at the Laurels. Mathilde was American in origin and had been a present from an American god parent to one of the children of the Bassingtons, a family which lived at the Laurels in the distant past. Mathilde was described as "Mathilde was a rather splendid-looking horse even in decay." It had once been painted grey. Of its mane, only a few hairs were left. One ear was broken off. Its front legs splayed out in front and its back legs at the back; it had a wispy tail. Isaac told Tuppence that "Miss Jenny", presumably a Bassington child, used to ride the horse everyday. Isaac himself used to ride on it some fifty or sixty years ago. As if to demonstrate, old Isaac mounted the horse again and rode it back and forth. It was still solid, he said, and it's "[g]ot action". The hollow “belly” of the horse is a significant plot element.
Like many other things at the Laurels, Mathilde was patterned after Agatha Christie's own rocking horse at her childhood home Ashfield which was also named Mathilde.