Tirtouga resembles a baby leatherback sea turtle. Its body is blue with black spots on the edges of its flippers. Its black shell has six circular indentations in it. It has a black upper beak and a mask that covers most of its head. It has a light blue patch underneath its eyes. Tirtouga's shell is very tough, and it can dive down to depths of as much as 1000 meters. Tirtouga was known to actively hunt for prey to the extent that it would go on land to reach them. Tirtouga lived in the oceans long ago and is now extant only by reviving it from fossils. The people of Alola believe it to be an ancestor of most turtle Pokémon.
Marshal owned a Tirtouga sometime before arriving in Nimbasa City. Because of his hard-to-control nature, he released him the day before meeting Black and White. He was later captured by Black and named Costa. Under Black's ownership, he eventually evolved. He first appeared in Growing Pains.
Tirtouga may be a corruption of tortuga (Spanish for turtle, tir having a similar phonetic pronunciation to the tur in turtle) or derived from Tortuga (translated as Turtle Island in French and Spanish). It may also be a corruption of tartaruga, Italian and Portuguese for tortoise, seeing as turtles are closely related to tortoises.
Protoga may be a combination of Protostega (an extinct turtle species largest second to only the Archelon, which its evolution is based on) and tortuga.