Um, you guys do realise that "thrasher" is a weapon type, not an animal type, right? Specific animals are not "thrashers" - thrashers have never been defined as creature types in Traveller.
Based on the MGT table (and supported by what was said previously in CT's Animal Encounter tables in book 3) I think a more accurate interpretation is that "Thrashers" are actually appendages that the animal thrashes around (think "Ankylosaurus", with its big club tail) - i.e. something where the animal throws its body weight around to do damage. So a heavy tail is an obvious thrasher - I guess a "wing bash" might count for a big enough winged creature, and if you really stretch the definition then maybe a "body slam" or "trample" would count too?
Also, if you look at the rest of the weapon table you'll see teeth, claws, hooves, horns, and stingers - the only class of "attack appendage" missing from that table are tails, so it stands to reason that as the one remaining item, "thrasher" must be tails (and other similar attacks).