All animals are either Beasts or Creatures. This is usually consistent for a given species, but not always; for example, most dogs are Beasts and most cats are Creatures, but occasionally one will encounter a cat who is clearly a Beast, or dog who's an absolute Creature.
I think it's kinda interesting how, while the Beasts are opposites of the Ancients, some feel like reflections to Ancients they're not directly related to.
Starting with the pair that feels most obvious, Silent Salt and Dark Cacao. Their designs especially seem similar with the armor, sword, and color palette. Silence also feels like it can be related to Dark Cacao's isolation during his previous arc.
This feels the same as with Pure Vanilla and Mystic Flour. Again with their color palettes, but also with their religious theming, both being obligated to help all those that seek their guidance.
Again, mostly similar vibe with these two. You could even argue that Destruction can be considered an extention of Passion in a way. These next pairs get a bit weaker with their comparisons, which is why I started with the strongest ones first.
My comparisons for Golden Cheese and Eternal Sugar come down to both of them having wings and some speculation I have. "Eternal" I feel could imply abundance, which may be a motivation for Eternal Sugar's slothfulness. It's possible they simply have all they desire, thus no longer having the will to do anything.
Lastly, for White Lily Cookie and Shadow Milk, the biggest point of comparison I have is that Shadow Milk Cookie's theatrical presentation could also be considered an extention of freedom (especially since it's implied that he used to be an academic). This is the one I really stretch the most with, I'd say.
Why do I bring any of this up? What I think about these possible links is that the Beasts reflect what each of the Ancients could become, if taken to their logical extreme. Also, with those that share their Soul Jams being their opposites (Pure Vanilla being truth and Shadow Milk being deceit, etc.), it can imply that each of the Ancients are each other's opposites as well. Opposites that mesh well enough despite it, anyway.
I'm bad at analysis, so do what you will with this. I don't know if this is anything. Implications and whatnot.
Thesis: while strictly speaking a "beast" is any non-human animal, archetypally a "beast" is a large quadrupedal vertebrate, and the further an animal is from embodying that archetype, the funnier describing it as a "beast" becomes.