The scenario you described is best explained by genetic drift.
In this situation, the animal's shorter tail does not impact its survival or reproductive success, which means that the variation is not influenced by natural selection. Instead, the fact that the animal survives and reproduces purely by chance (or luck) leads to the propagation of the shorter tail trait within the population. This random change in allele frequencies due to chance events is characteristic of genetic drift.
So the correct answer is genetic drift.