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.
The game's lore explicitly states two things:
- Boss Monsters' SOULs persist briefly after death.
- Boss Monsters only age when their offspring does. This explains Asgore and Toriel's longevity as their son, Asriel, died at one point, and they therefore cannot biologically age.
When a Boss Monster dies, their body turns to dust; however, their SOUL, in an upside down heart shape, lingers for a few seconds before shattering. Similar to other monsters' SOULs, the souls of Boss Monsters are white.
There are currently three known Boss Monsters: Toriel, Asgore, and their son, Asriel. Although not necessarily 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, paws at the end of their extremities, and an affinity for fire magic. All currently known Boss Monsters are also part of the same royal bloodline. Boss Monsters rule over the Underground, which likely explains their name.
- The goat theme may very likely be a deconstruction/reconstruction of typical devilish traits, as devils often have goat traits and goats are used prominently in Satanic symbolism. However, this is made mundane since Boss Monsters are essentially anthropomorphic goats. Many powerful RPG bosses are devilish, hence the name and significance also back this up.
- The fact that they all can use fire magic and Asgore using a trident also supports this, as the devil is often associated with fire and the trident.
- A Boss Monster's SOUL is strong enough to persist after death... If only for a few moments. - Inscriptions on the walls of Waterfall
- We call 'em Boss Monsters. When they have an offspring, the SOUL power of the parents flows into the child... Causing the child to grow as the parents age. But ASGORE doesn't have a child. So he's been stuck at the same age... And probably will be forever. - Gerson