No - the kind of abnormality you described wouldn't be the result of evolution, which involves changes in a whole population, but of a mutation in an individual. Such an abnormal individual is called a 'sport' ("an animal or plant showing abnormal or striking variation from the parent type, especially in form or colour, as a result of spontaneous mutation" Oxford Dictionary).
Such a mutation in an individual, even if heritable, would be unlikely to persist more than a generation or two at most, so evolution wouldn't be involved.
It's also worth noting that although evolution generates all species, prey species wouldn't evolve as provision for some new predator species, one-legged or otherwise. Co-evolutionary predator-prey development goes the other way, because the predator (by acting as the natural selection pressure and weeding out the easiest prey) drives the evolution of the prey species to be more difficult to prey on. This in turn drives the evolution of the predator to be better at predating the prey. This is often called an 'evolutionary arms race'.
Fossilization is rare, and finding fossils even rarer, so while we may occasionally find a few individual fossils out of a large population or whole species (e.g. tens of thousands to millions of individuals), the chances of finding fossils of individual 'sports' are infinitesimal, and since soft tissues aren't usually fossilized, such a fossil would only be recognised as a 'sport' if the mutation resulted in significant bone malformation.
For less severe heritable mutations, the likelihood of finding a fossil of them will be proportional to their prevalence in the overall population at the time and place that discoverable fossil beds were laid down for that population (if at all).
However, fossils showing evidence of diseases caused by mutations (e.g. tumours) have been found; these mutations mainly affect somatic cells, so they are not usually heritable.