Silkie is a moth larva the size of a housecat thanks to Killer Moths experimentation into weaponized moths. Silkie was found initially by Beast Boy who hid him from the others, knowing Robin wouldn't allow pets in the Tower.
When Silkie started to eat various objects lying around the Tower, including some personal belongings of the others, Beast Boy persuaded Starfire to hide Silkie for him, when Starfire saw Silkie, it was love at first sight.
Silkie's eating habits continued to be a problem as he just grew and grew, eventually becoming larger than the T-car, which it had also taken a bit out of and Robin asked Starfire to release Silkie into the wild.
Killer Moth found Silkie soon after and forced it to undergo the metamorphosis into a gigantic moth that he could ride. Because of Silkie's vast size the Titans soon met the pair and a battle ensured, where Moth revealed that he had noticed Silkie's absence and that he had no idea what they had been feeding it to make it so big. In the end Starfire persuaded Silkie to turn on Moth because of how he abused it, making it crash to the ground turning it to pink goo with the impact.
Starfire was distraught initially over Silkie's demise, before she found Silkie having returned to larva-state. The two then proceeded to eat the goo-remains, much to the disgust of the other Titans, since the taste appealed to Starfire's stomachs.
After this adventure, Silkie remained as Starfire's pet and for the most part only made cameo appearances, such as being one of many objects that were thrown out of a closet Beast Boy was looking through, appearing only to eat something none of the others would.
In the companion comic to the show, Silkie did save the day of the final issue, where all of the Titans had been brought down by Phobia and trapped them in their own nightmares. While Phobia was busy reporting her success to Brain and Mallah, Silkie rolled onto the alarm, which woke the others up.