One correction to the translator: It's not Rock Goblin, but a "ROKUBLIN", or a "6blin", It's a joke based on the number 5 being read as "GO", so once a 5blin evolves, it becomes a Rokublin (6blin).
This is in preparation for a future joke where we meet the "Mangoblin", a Goblin that went through evolution 10005 times, and is treated as "a goblin with a Manko", manko being the japanese for pus*y
Also, in the same Page 7, this explains Sao's questioning his name, he is literally counting numbers with blin:
1 = 一 (ichi) + blin = Ichiblin
2 = 二 (ni) + blin = Niblin
3 = 三 (mi) + blin = Miblin
4 = 四 (shi) + blin = Shiblin
5 = 五 (go) + blin = Goblin
6 = 六 (roku) + blin = Rokublin
The reason Sao is guessing is because the names are in Katakana (no Kanji), so once he hears the name "rokublin", he could only imagine that Goblins for some reason must start from "Ichiblin", evolving until the commonly known Goblin, and then a "Rokublin", but it's all just a guess, he also imagines there would be a Nanablin (7) and Hachiblin (8) in the future if he is correct.