Orcs.
The most numerous enemy within Tolkien's legendarium,
orcs are a common enemy for all the free peoples of Middle-Earth to battle against,
and their savagery and barbarism knows few limits.
While orcs certainly do not have the history or importance of other races in Middle-Earth,
there's still some discussion to be had pertaining them.
The origin of Orcs is still unclear to us, as Tolkien died before he could fully cement their history,
but there are a few theories.
The prevailing idea amongst most readers is that orcs were originally elves and/or men
that were corrupted by Morgoth.
This is the most likely origin as presented by Tolkien
but it's unlikely that all orcs are corrupted men,
as the first orcs appeared before the First men.
Other theories include orcs being soulless animals of vaguely human shapes that Morgoth empowered
or even that the first orcs were fallen Maia like Balrogs.
Regardless, there's no true origin of Orcs that can be clearly presented,
so it remains up to reader interpretation.
Orcs first appeared during the time Morgoth controlled Middle-Earth
and they fought in the war against the elves and the Valar.
Although Morgoth lost the war and was captured
some orcs did manage to hide and multiply while waiting for their master's return.
Upon Morgoth's return and throughout the First Age
hundreds of thousands of Orcs were bred for war and they formed the core force of Morgoth's armies.
In the War of Wrath at the end of the First Age,
nearly all the orcs were wiped out, but some managed to flee eastward.
During the Second Age, when Sauron reappeared and took over Mordor
he began gathering and breeding his own army of orcs.
Initially, these orcs did not see much combat, as Sauron sought more subtle means of controlling Middle-Earth,
by creating the Rings of Power.
Eventually, Sauron became much more direct with his enemies
and used orcs as the main power of his armies much like Morgoth did.
Despite their sheer number, however Sauron and his orcs were still defeated
by the combined might of the elves and the Numenoreans.
A group of orcs, acting independently after the War of Wrath
had moved towards the Misty Mountains and began attacking people in nearby areas.
This included the dwarves of Moria, the rangers of the North
and some even reached the Shire, but were stopped by a group of hobbits, led by Bull-Roarer Took.
The orcs of the Misty Mountain led by a chieftain, named Azog
would later kill Thorin Oakenshield's grandfather, beginning the first war of the dwarves and orcs.
Although the orcs of the Misty Mountain were mostly wiped out during the war,
some remained and years later would encounter Thorin and company.
During their escape, they managed to slay the leader of the orcs
and the orcs followed them, seeking retaliation.
This led to the Battle of Five Armies near Erebor,
with the orcs joining forces with orcs from the Great Mountains,
as well as wargs and large bats to combat a group of elves, men and dwarves.
The orcs lost the battle with three quarters of the population being wiped out,
leaving the Misty Mountains free of orcs for many years.
Unfortunately, when some dwarves later attempted to reclaim Moria,
they were ambushed by orcs and killed.
When Sauron had returned to Middle-Earth after the fall of Numenor
orcs once again comprised the majority of his forces and once again lost during the War of the Last Alliance.
A group of orcs from the Misty Mountains ambushed and killed Isildur,
while he held the one ring, leaving the ring at the bottom of a river.
Throughout the Third Age, orcs were bred and used by the Witch-King of Angmar,
chief of the Nazgul, as well as Saruman and, of course Sauron.
Saruman had managed to create a new breed of orc, named Uruk-Hai,
possibly by crossbreeding orcs with men.
These orcs were stronger, faster and larger than normal orcs
and were the main force at the battle of Helm's Deep.
Orcs fought in many battles throughout the War of the Ring
and although Sauron eventually lost the war,
it's unclear how many orcs survived into the Fourth Age.
Very little is said of their history during the Fourth Age, other than they must have fled
or been defeated in Moria, because the dwarves eventually reclaimed it.
Physically, orcs varied in appearance, but most of them had long arms and fanged mouths
with some having black skin and some having short crooked legs.
Notably, they were almost all shorter than the average man
with many of them apparently around the same height as hobbits.
Their lifespan is unknown,
although we know some orcs to be at least 150 years old.
If they are indeed corrupted elves,
it's possible some orcs are immortal like the elves, but this is unproven.
It is important to note that although the term "goblin" is occasionally used throughout Tolkien's works,
the term is used interchangeably with orc and does not represent a separate species.
Orcs are extremely warlike and were fond of using poison weapons as well as practicing cannibalism.
Orcs possess no language of their own,
instead using a mixture of various other languages,
although during the Third Age, Sauron created Black Speech
in an attempt to unify the language of his servants.
The language didn't catch on entirely with the orcs
but they did borrow various terms from it.
Although it's very easy to just view orcs as expendable troops used for their innate savagery and bloodlust,
Tolkien regarded them as not evil in their own right
but merely as tools for Morgoth and Sauron.
If they are corrupted men and elves
it makes sense that much of their free will and original personalities would be altered.
Even though there's no orc in Tolkien's works that could be considered good,
other pieces of fiction would be strongly inspired by Tolkien's orcs
and offer the potential for different types of orcs.
This ranges from Dungeons and Dragons, where players could potentially play as good orcs,
to the Warcraft universe where many orcs are rather civilized creatures.
While Tolkien's orcs are somewhat limited in their scope and potential,
once again he paved the way for many years of fantasy fiction to come.