The unknown creature is an animal that features in the epilogue to Dougal Dixon's After Man: A Zoology of the Future, Dixon speculates on what directions life could possibly take after the in-depth view of 50 million years hence. He argues that the history of animal life was largely dependent on the history of plant life, and that new animal forms developed after new plants, and so tries to invent a new sort of plant and an animal associated with it. The plant he proposes is flowering, and bears fruit, but does not fall to Earth when ripe, actually growing on the herbivore until the beast removes it and puts it on the ground, where it grows into another tree.
The animal itself is of unknown affiliation, as was the intention of Dixon; it cannot quite be placed as either mammal, bird, reptile, or amphibian, and its ancestry is also not mentioned. Dixon himself only lists that it may be descended from living generalist lineages like the crows or "insectivores" (an archaic classification for primitive-looking placental mammals). The animal is covered with what looks like a thick intergument but could possibly be scales or ridges, and unlike most tetrapods only has two limbs. Unlike most of Dixon's animals, he does not provide a scientific name or detailed analysis of the animal.
Although Dixon's unknown creature may seem bizzare, it does make a valid point; evolution into the far future can be incredibly difficult to predict. Imagining a new class of vertebrate that does not fall into one of the existing forms is difficult; even more difficult would be to imagine some new phylum. Speculation on the order of hundreds of millions of years is very challenging, as this creature points out.