In the words of Nicholas Sekunda, "The strike force of Hellenistic armies continued to be elite heavy cavalrymen" (344). Heavy cavalry featured in large numbers in all major Hellenistic battles. At Magnesia, Antiochius commanded 3000 cataphracts and 1000 elite cavalry forming the agema, as well as nearly 10,000 light infantry (Livy 37.40). At Cynoscephalae, Philip V's cavalry had done a fair bit of fighting in the campaign to that point (Livy 33.6), but the effectiveness of his cavalry was greatly limited by the superior Roman numbers of high quality Aetolian cavalry, who were allegedly the best in Greece at the time (Livy 33.7). However when Philip was observing the battle, he would have been accompanied by his personal guard, the agema, at a very minimum. Hellenistic monarchs such as Antiochus and Philip were renowned cavalry commanders, and tended to lead heavy cavalry charges in battle, much as Alexander did (the Battle of Raphia for instance: Pol 5.80-6).

However, Alexander's army when he crossed into Asia had a particularly large infantry to cavalry ratio (32,000 inf, 5,100 cav), but in the Hellenistic period armies became much, much larger and it was difficult for Hellenistic states to afford to maintain such high cavalry ratios (it was more like 1:10 at Raphia for both the Ptolemaics and Seleucids), and the phalanx now became the crux of battle plans (Sekunda 347). There was also a decline in the quality of cavalry during this period, not only in the fighting quality of individual cavalrymen but also their tactical proficiency, such as use of rhomboid and wedge formations, and they were generally used in more linear and deeper formations in the Hellenistic period (347). However, despite the quality of Alexander's cavalry (like all components of his army), they suffered disproportionate casualties (Bosworth 270), and after only a few years in Asia/Persia Alexander was having to make major reorganisations of his cavalry units. The quality of Alexander's army was something that had been honed over a long period by his father, but huge casualties seriously blunted this instrument in a fairly short space of time, with Macedonian manpower (the source of his best cavalry) being particularly overwhelmed (267). He may never again have commanded an army of the quality he left Macedonia with in 334 BC, and it is perhaps little surprise that his Successors had even greater trouble raising enormous numbers of good quality troops.

Essentially, the Companion Cavalry, the agema and other types of cavalry upon which Alexander depended continued to exist in a less-effective form through the Hellenistic period. Heavy cavalry participated in large numbers in all major Hellenistic battles as far as I am aware, certainly against the Romans. It was not their raw numbers but their numbers relative to the enormous numbers of infantry in Hellenistic armies that declined. Combined with the more static tactics of the phalangites in the Hellenistic period (as they were also of a generally lower quality than under Alexander) and the decline in the quality of the cavalry themselves in terms of individual and unit skills, the cavalry formed a less decisive component of Hellenistic armies than they did under Alexander. However, they were still there, and they were still important.

Sources:

Livy, Polybius (Loeb translations)

Bosworth, A.B. Conquest and Empire: The Reign of Alexander the Great. Cambridge: Cambridge University Press, 1993.

Sekunda, "Land Forces", from from The Cambridge History of Greek and Roman Warfare Vol. I, Eds. Philip Sabin, Michael Whitby, Hans van Wees. London: Cambridge University Press, 2007 325-357.