This could be a bit of a stretch, but I still think what I’m
about to interpret engages the material in an interesting way. You can take it
with however many grains of salt that you wish. Remember that there can always
be both a figurative and literal element of what characters and objects symbolize – it’s not mutually exclusive.

Before I get into what America’s pets represent, it’s
important to explain why America can see aliens, but not ghosts. This often
confuses a lot of people, but if you refer to the strips, an implicit but
nonetheless consistent logic is used.

England can see ghosts and fantastical creatures because
approximately 70% of his people believe in them. At least, this is the figure
provided in the strips, I can’t attest to how accurate that is. What’s most
important is that this is used in the reasoning behind why Hima characterizes
England to possess these abilities.

Since 23% of Americans believe in ghosts, America is unable
to see them. Although, admittedly, this does seem to change as the strips
become more recent along the timeline. [I’ll get into this in another post].

Similarly, because Americans do not believe in the fantastical
with the small exception of Halloween, this is the only time where America is
able to see magical creatures and ghosts.

Now, given that UFO’s are only “typically seen” in the US,
it would follow that Americans possess stronger beliefs in aliens than they do with
the supernatural. This would explain why America is friends with Tony,
meanwhile England – whose people don’t possess such a high belief in aliens –
is skeptical of Tony.

So…what do America’s pets potentially represent?

It’s not an uncommon trope for authors to make surrounding
objects, people, or pets represent aspects of a character’s mind.

America’s Pet Whale: 

In that case, while America did go whale-hunting in Japan
for commercial purposes, the whale he later befriends and adopts could also stand
for his gluttony.

This meaning doesn’t only limit itself to America’s gorging
appetite either.

It can also stand for America’s expansion and befriending of
other nations at the expense of leaving England behind and turning his back on
him.

Ex: When England hears that America is attempting
to befriend a whale, he shyly offers to become his friend.

You can only guess what happens next.

Tony: 

If pets, or rather, familiars, represent the inner framework
of a character’s mind, then Tony, as a being of space, could also be equated to
America’s own headspace.

In other words, Tony seems to embody America’s old grudge
towards England.

Notice how when Tony and England first meet, Tony calls
England a “F***ing limey.” 

Meanwhile, America stands there uselessly and is
unable to read the situation…

…or rather, what if he is purposefully repressing and
keeping his emotions in check, only for Tony to be the one to express them?

Alternatively, Tony could be reading America’s emotions
without the latter realizing this. It’s not entirely impossible given that
aliens in popular culture often have telepathy or some other kind of
mind-reading power. In that regard, maybe Tony has read America’s mind/memories, and is reacting with hostility towards England because of this…

Consider the whole response now: England asks Tony about his home planet.

“Not going back! F***ing Limey!” could likewise be
interpreted as America’s vehement opposition to the mere thought of being under
England’s rule again. It almost seems as if Tony’s acting as America’s
unconscious mouthpiece…asides from the death threat.

We see this again when England visits America and complains
that there’s a ghost [it’s actually France] in his house.

Notice the difference in reactions between America and Tony.
America laughs it off, while Tony appears to taunt and belittle England for
being easily frightened as if he were a small child.

On the other hand, Tony has been seen to end his sentences with “bubu” so this could just be a misspelling. 

More on America’s Emotions: 

I’ve also noticed that America’s pets reciprocate his
affection for other nations.

Ex: They are
exceptionally fond of Lithuania.

Ex: When
Lithuania falls back under Russia’s control during the Great Depression,
America seems both worried and sad to see him go.

In sum, while America’s pets are real, there does seem to be
some evidence substantiating the possibility of them also representing aspects
of his inner psyche.