Mylortherium, "Peaceful Beast", is an ancient species said to be a pyroshark.
Old players who saw Mylotherium say that it was NOT a Pyroshark it was a species of standard shark. With a TF2 Graphics style to it. It was classified as an official TF2nimal and the first of the Realoi Family.. People say it was peaceful and was a herbivore.
Probably deletion from the TF2 world by Lord Gabens power. It served no use to the ecosystem. Others suggest it was killed off by ocean predators...