Ferals are dragons only in the most technical sense of the word. From coloration to temperament to behavioral patterns, they have little in common with other dragons. Their difference is apparent from Hatching forward. Tending to be larger than standard dragons, they come in entirely different colors: Solar, Light, Lunar, Supernova, Nebula, Twilight, Dawn, Daylight, and Storm varieties do not appear anywhere but in ferals.
Important to note is that, in the absence of Thread, the line that produced the ferals seems to have reverted somewhat back to features of their fire-lizard ancestors. Like fire-lizards, ferals do not necessarily have to Impress, though those who choose not to are prone to mental instability, and are fully capable of leading lives independent of humans (unbonded ferals do, however, tend to have similar lifespans to their bonded brethren - few live long past the lifespan of a human). Also like fire-lizards, they are fairly able to care for themselves from Hatching, able to visualize and between
as well as flutter and glide - through true flight is not really possible for some time - the moment they are free of the shell.
Ferals also differ from standard dragons in their behavior. While to a normal dragon inflicting harm upon another or a human is entirely unthinkable under any but the most dire of circumstances, ferals are more than willing to fight each other and to kill humans, though none but the most disturbed will do so without good reason. This is, of course, not to say that ferals are entirely without scruples: it is rare to encounter a Feral readily willing to harm or kill a human bonded to a dragon, harm a hatchling or any sort of child, or inflict unnecessary pain. Though some colors are more volatile or docile than others, the range of "good" and "bad" in ferals tends to be similar to that in human beings.
Another unique feature of ferals is their preference in bondings. Like normal dragons, those ferals who Impress invariably do so directly upon Hatching. However, they do not Impress humans in the average age range of about twelve to twenty-one. ferals only Impress to young children, usually between the ages of six and fifteen (as mostly of these child-candidates have not yet determined their sexuality, ferals choose Theirs, if they do so at all, without discrimination to that regard, which can get confusing later in life, but generally doesn't cause too many problems). This results perhaps from the unique independence of ferals, who prefer to be the dominant personality and the guiding individual in their bondings. Whatever the reason, to facilitate this preference ferals have developed a unique ability: unlike normal dragons, ferals are able to block the emotions of their Flights from their young riders until they are old enough to handle them. Of course, these suppressed Flights are generally shorter and more strained, and thus produce far fewer eggs, if any, than otherwise, but they serve the purpose of protecting the child-riders from being physically, emotionally, or mentally damaged by Flights that come too soon.
Note: Almost all of this is unknown to the rest of Pern, save that these dragons come in separate colors and choose young riders. For a compilation of known information on ferals (known to non-feral characters, that is), see this thread.