There's three kind of Mudcrabs.
The Smallest ones have Flat shells, are the closest anology to the classic Mudcrab.
The Larget mudcrab, that has more of a conical shell are quite dangerous at lower levels, especially on master.
The largest Mudcrab appears to have a horn structure, and the crab itself is larger than an Imperial or Bosmer, they can hit upwards of 50 Damage on an unarmored person.