Boss Monster

A Boss Monster is an anthropomorphic goat-esque type of monster which has the differentiating trait known as SOUL persistence. Although all Boss Monsters are also bosses in Undertale, not all bosses in Undertale are Boss Monsters. All Boss Monster encounters activate Serious Mode.

Overview
The game's lore explicitly states two things: When a Boss Monster dies, their body turns to dust; however, their SOUL, in a heart shape, lingers for a few seconds before shattering. Similar to other monsters' SOULs, the souls of Boss Monsters are white.
 * 1) Boss Monsters' SOULs persist briefly after death.
 * 2) Boss Monsters only age when their offspring does.

There are currently three known Boss Monsters: Toriel, Asgore, and their son, Asriel. Although not intrinsic characteristics of Boss Monsters, the three of them share several traits; namely, a resemblance to Nubian goats, long droopy ears, visible fangs, a pair of horns, anthropomorphic bodies, and paws at the end of their extremities. All currently known Boss Monsters are also part of the same royal bloodline. Boss Monsters rule over the Underground, this likely explains their name.