There absolutely was a history of female warriors in neighbouring countries. The Amazons were originally palace guards and the idea of all female palace guards was relatively common in West Africa (and not just West Africa, the Kingdom of Haiti, ran by African slaves, had female palace guards) due to the idea that since women couldn't become King, they'd be more trustworthy. The reason why the Amazons are unique is that they were armed with guns and sent to conquer foreign cities which the other female guards tended not to be.

In terms of them officially being wives of the ruler, yes but that meant a lot less than you might think. Fon society was about lineages, you had loyalty to others of your blood lineage . So when the King of Dahomey conquered a new area he married a member of each new lineage to make his family include every family of his subjects and so tie their lineages into his own. He often had thousands of wives, some of which were literal children and some of which were actually men because no women were available. In practice 4 or 6 would serve as actual wives and the rest would be used in various jobs. The wives would often make up the government and the bureaucracy, during the early 19th century they were often the most powerful people in the country. They would also make up the government funded prostitution service which were deliberately provided at low prices so even the poorest peasants could buy sex.

During the late 19th century, the power began to shift from the palace women, who traditionally picked the new King to traders outside the palace (there were no rules of succession, instead the heirs would fight it out for who would win by gaining allies among the powerful members of the government). The last three kings were picked against the will of the palace women because the traders had become more powerful than the palace and so women within the palace became less powerful and more subservient to the kings. The palace guard thus became less powerful and more subservient so became the nucleus of a slave army, the amazons. Women captured in raids were sent to the army to be trained (men tended to be sold instead because they would get higher prices).

Now there are various reasons given as to why Dahomey armed it's female guards with guns and used them as front line troops which nobody else did. The most plausible is simply that Dahomey was a country designed for expansion, it's religion was built around capturing human sacrifices in war, it's economy around capturing slaves to sell to Europe and it's political legitimacy around conquering new lineages. More to the point, the traders grew rich traditionally by sponsoring soldiers and thus earning slaves who they could sell. The only way for riches was by war, so when you don't have enough men, you arm the women instead. And for the palace, using the palace women meant all the slaves they capture belong to the palace so it's the King who gets richer.

I've seen it said that the fact they were female elephant hunters convinced the King that women could fight but by all accounts they were no Elephants in Benin during this time period so I'm sceptical.

Also the exact nature of gender in Dahomey is debatable. Men, both eunuchs and non-eunuchs, were given feminine roles and clothing by the king and so were addressed by feminine pronouns. Hence the Kings' male 'wives'. While in the other way, female soldiers sang songs saying ‘we are men, we are not women’. It is possible that rather than a gender binary decided by sex, Dahomey operated one decided by role and the palace women who became soldiers were ‘promoted’ to men. However, this is something difficult to prove either way and, at least, by the 1890s those female soldiers were primarily described internally as female. Culture is a moving target, it may be true that the link between sex and gender became more cemented as Brazilian, European and Yoruba educated figures took over from Fon figures in the positions of power.

Also the increased status of female military units, came hand in hand with the loss in power of palace women during the late 19th century, essentially women were moved from being movers and shakers to being loyal servants. A soldier was a more subservient position than a government minister. But subservient didn't mean un-appreciated. The most feted people in late 19th century Dahomey were the Royal Army and the Amazons, with peace time rulers claiming positions in charge of Amazon battalions that they never held in order to borrow prestige and military parades forming the centre of any celebration.

The military worship of late Dahomey, which came from a type of nostalgia to the days of the peak of slave trade and Dahomian conquest, gave the army an importance it did not have in earlier days when it was used more. The Amazons were a relatively late innovation compared to the women’s government in the Palace, dating probably only from the 19th century. They began as a primarily Fon force, an extension of the existing female palace guard but under Gezo (King from 1818-1858) it became a slave army under direct control of the King. Its recruits were taken from war captives and those outside the existing hierarchy to ensure their loyalty and they were made to kill other captives in order to blood themselves. This also increased the wealth of the King by ensuring all war captives were property of the king himself due to being captured by his personal guard and not soldiers who owed patronage to a non-royal backer.

Ironically as worship of the army became more common, its effectiveness declined. Dahomey was able to expand to such a large extent as it did in the 18th century because it was surrounded by hundreds of tiny independent polities, villages and city states. As it expanded, it began to encounter larger entities which hemmed in its growth.

Being unable to sustain itself through breeding, the system required a steady stream of new captives and as conquest slowed it became unsustainable and needed reform. Under Gezo’s son, Glele, every peasant family was obliged to give a daughter to the King to become an Amazon, in order to replace the reduced amount of captives.

While in the 1890s women were at their most visible to visitors to Dahomey, thanks to the military, the guards and the household of the palace, this hid a worsening position for women generally. On the individual level, punishments for female infidelity increased during this time, with the Vodun priests in particular becoming more hostile to women, in some cases building shrines decorated with the genitals of dead women. And on the national level, women found themselves removed from real power in the palace and more likely to be forced into subservient roles, state sponsored prostitution increased during this time period as did female slavery in the palm oil trade due to superstitions about it being unlucky for men to work in the palm oil slave plantations which increasingly grew in the late 19th century in Dhaomey. The Amazons were as much a sign of the feminist credentials of the later Kings as Peasant levies proved the socialist credentials of the Ancien Régime.

This was a culture, which had a history wherein an elite of women had power but women outside of the palace were still held as inferior in the eyes of the law despite that power. For example, divorces existed, which was an unusually feminist situation, but could only take place at the father of the bride’s request, not at the bride’s. In many ways, they faced a reversal of the usual West African situation, where centralised control weakened the role of women as they held power on a local level but not on a national one, in Dahomey the role of women became weakened the more the centralised monarchy lost power to influential local traders, who themselves being Yoruba or Brazilian educated often held more negative views on women in power. But that's a simplification in itself because, especially in the periphery of royal power, some of those local traders were themselves women.

Source: Various but primarily Wives of the Leopard: Gender, Politics, and Culture in the Kingdom of Dahomey by Edna Bay