As a primer, remember: you are effectively a fully-formed existence that's a newborn. You have knowledge about yourself and concepts, but no history. In addition, you might not know anything of the world, but it is not familiar or unfamiliar. It is merely natural--something that you would not give a second thought about outside of philosophical thought.

Name
The name is more or less self explanatory. Boss monsters (re: all of you) are born with an inherent sense of self. This includes the right to name themselves. The source of the name doesn't truly matter--it can be ingrained in your soul or it can be something as arbitrary as looking around you and looking at a pea, a tear, and a gryphon and liking how they sound together. Though please don't do exactly that.

Also put your title in your name, if you have one--IE, [name] the Goblin Lord or [name], Deep Dark Dragon of Destruction.

Rarity
Replace this number with 1★ through 5★. This isn't entirely a permanent decision and should be associated with your race. Rarity can be increased via special materials. Think of this as 20% your starting stats (these are arbitrary and do not mechanically exist), 40% your growth curve, and 40% your character's endgame potential.

What benefits are there for weaker ranks? Without committing to specifics or small spoilers, lower rarities will advance faster (make up for their worse stat growth) and may or may not be less frightening for some of the mooks that will populate your dungeon. But again, arbitrary.

As for rarity, you can imagine it as a curve. 1★ would be something like a goblin chief. 5* would be something like a dragonlord. Rarity is either an evolutionary or rank path. Using goblin chief as an example, it would go goblin chief > lord > baron > king > emperor for a rank-rarity. For evolutionary, imagine goblin > hobgoblin > orc > ogre > oni.

Do stars beyond 5★ exist? If I think it's interesting in the future, it will.

Note: this is primarily an aesthetic and thematic device. It, like most things, are entirely arbitrary.

Level
Replace level with 1.

All fresh spawns are level 1. This means the average adventurer will kick your ass as it stands, even if you are a 5★ Titanocider Ragnarok Behemoth. This serves the thematic purpose of actually needing this dungeon. Note that acquiring dungeon points (see: having adventurers die inside) will strengthen your dungeon core. This is the source of your initial levels--making the dungeon better will advance you all about equally.

Appearance
Things that your image doesn't cover. Write whatever, but some nice things to include: size/scale, impression, and things like that.

Personality
How your character acts. Be sure to include their desires--all monsters fundamentally have desires that put them in opposition with others.

Abilities
Put in your starting set of abilities. Skills, magic, abilities, etc. Consider this as generally more free-form than a rigid list (though having a few rigid techniques that gain power in exchange for flexibility is fine) Remember, combat isn't a giant part of the RP. Consider this through the lens of utility and narrative possibility (see: humanization, disguising, crafts) in addition to any power fantasy.