Reptar began appearing in Greg's videos in 2013. Reptar was more than likely named after the cartoon dinosaur from the 90's cartoon Rugrats.
Greg featured Reptar in a few Onision videos.
Reptar had also made a cameo in "Dog Thoughts 4".
Another concerning moment for Reptar was on April 3, 2013, when he became stuck between a rock and the side of his tank. Rather than helping Reptar right away, Greg posted a photo of him to Instagram: "Reptar got stuck and passed out on a rock... LOL." (Source)
DeathDobs and Leelu, were also in the backyard, Greg made the decision to leave Reptar under a plastic bin so the dogs would not bother him. A few hours later, Greg returned to the bin to find that Reptar had passed.
Greg announced Reptar's death with an Instragram photo of the bin Reptar dies under.(Source) Later that day, Greg posted a picture of himself mourning Reptar's death next to his tank. (Source) Greg also uploaded a video that day about Reptar's passing to his Archive channel titled, "Sad News". ($ource)
These two Instagram photos and the video set off a series of events that lead to what is known as Turtlegate. For more information, go to the Turtlegate article.
Greg has since deleted both of these posts from Instagram and Tumblr.