A skull is the shell of bone found within a beast’s head; the default skull is a human skull, and is also called a “death’s head”, especially when drawn without the lower jaw. It’s found in the arms of Bolter, 1632 [Guillim2 161], but is more famous as the attributed arms of Death [Neubecker 222].
There’s also a period example of an animal’s skull: the cow’s skull in the canting arms of Capo di Vacca, c.1550 [BSB Cod.Icon 275:23].
Jeremy of the East bears: Argent, a skull sable.
Calam Stiùbhard bears: Per pale gules and argent, a death’s head counterchanged.
Bodo Rosti bears: Quarterly sable and gules, in bend sinister two jawless skulls argent.