In the DMG section on "Bringing Back the Dead" (chapter 1, p. 24), it describes what happens when a creature dies:
When a creature dies, its soul departs its body, leaves the Material Plane, travels through the Astral Plane, and goes to abide on the plane where the creature's deity resides. If the creature didn't worship a deity, its soul departs to the plane corresponding to its alignment.
So what happens with the soul of something like a frog which is unaligned? Is there some plane corresponding to "unaligned", like the Outlands which correspond to neutrality? Does the soul get "stuck" in the Astral Plane when it can't go "to abide" elsewhere? Or do unaligned creatures somehow not have souls, or maybe they all worship a deity?
I'm aware that a DM can build the cosmology of their multiverse however they want (and the DMG specifically encourages such), and could make any reasonable decision here, even when using existing published settings. I'm just curious if there is any "official" or published guidance, both from a mechanics perspective (for things like raise dead where the soul apparently needs to be willing and at liberty to come back), and from a lore perspective (like is there some plane with a ton of animal souls while most Outer Planes don't have any?).
For published lore, I'd prefer knowing if there's any information about the handling of unaligned souls within the Forgotten Realms just as that's the published multiverse I'm most familiar with, but if other settings specifically tackle this question that would be interesting as well.
Best Answer
The rules on this are not specific in D&D 5e, leaving it up to the DM.
There's no particular rule that says one needs an alignment to have a soul. In fact, Dungeon Master's Guide p.24, "Bringing Back the Dead", would imply that all living creatures have souls which depart upon death, as cited in the question:
These are the only two rules in the books that specify where a soul goes after death: the plane of their deity, or the plane of their alignment.
However, according to Player's Handbook p. 122, "Alignment in the Multiverse", unaligned creatures do not have an alignment at all:
Assuming that frogs don't have a deity either, this means that there is no rule to cover this circumstance. All we know is from the first line of the "Bringing Back the Dead" quote:
The spell description of raise dead (PHB p.270) states that it works on any dead creature (other than undead), which implies that frogs can be raised, suggesting that their souls go somewhere from where they can be recovered. Dungeon Master's Guide p.43, "Putting the Planes Together", suggests that most campaigns require a place where mortal spirits go, which would in general include unaligned creatures.
Lore from previous editions of the game
The Great Wheel cosmology is described in the D&D 3rd edition version of the Manual of the Planes. On p. 141, it asserts that some animals may go to the Wilderness of the Beastlands:
Page 199 of this book asserts that while it's entirely up to the DM, by default any creature with Intelligence and Wisdom scores of 1 may become a petitioner, a soul inhabiting the Outer Planes. Animals are very specifically stated to qualify for petitioner status. This is partly because D&D 3e generally asserted that animals were True Neutral in alignment, rather than Unaligned.