"Cossacks: when did they change? " Topic

I understand that in the 16th and 17th Century Cossacks were largely foot troops. By the 19th Century they were renowned as light cavalry. How did that come about?

Cossacks were a mix of peoples. Read all about it!
link

True story

Cossack communities often were at the edges of the Russian Empire, and as the Empire expanded in the 18th and 19th centuries the Tzars called upon them to furnish troops, mostly light cavalry; by the time of the French Revolution/Napoleonic Wars this was regulated by the Tzar and the only troops raised in most Cossack hosts were cavalry

Have a few battalions of foot Cossacks in my Napoleonic era army

When the Cossacks were formed as an organized force (XVI and XVII centuries), the main type of battle that they fought was fighting with firearms. This type of battle gave them an advantage over the nomads, and allowed them to successfully fight against European opponents.
As the firearms spread, this advantage over the eastern enemy lost its value. Since the Cossacks, mostly settled in the flat areas and carried the border service, suppressing the raids the nomadic tribes on Russia, it was more appropriate for them to serve in the equestrian system.
In the Napoleonic era, there were no foot Cossacks (as representatives of the military class). "Cossacks" be called part of the militia, created from ordinary citizens and peasants. Here the word "Cossack" was used in the meaning of "irregular troops".
Later, the pedestrian units of the Cossacks were created in the Caucasus, they were called "plastun", and served as intelligence officers and light infantry during combat operations in the mountains.

I read somewhere that the Cossacks made their start as river pirates and thus didn't have much use for horses in combat. I don't know if it's true.

The Cossacks did act as pirates very widely in the 16th and 17th centuries. Since the waterways were the main communications in the "Wild Steppe" and in general on the territory of Russia, the road network has been developed very poorly. In addition, they regularly made sea raids on the coast of Turkey. So they were also good sailors.
They were a good light cavalry in fact, but the steppe cavalry was still more effective, so the Cossacks preferred to fight against it with the help of a mobile fortified camp of carts. Here is an episode from a Polish film, where Ukrainian Cossacks lure Polish cavalry to the attack of a similar camp.

YouTube link

By the way, during the conquest of Siberia, the Cossacks Yermak also acted in the march on large boats in the form of a huge river flotilla. A bit of music and a clip in which you will see the Cossacks, both as river pirates, and as a "river" fleet when conquering Siberia.

YouTube link

Cuprum, thank you for replies. Found them very informative and useful.