The turtle/tortoise note is correct for UK English, but in the US (where Id software was headquartered during the development of Quake), tortoises are considered a subset of "turtles". Per Webster:

> Turtle (noun): any of an order (Testudines synonym Chelonia) of terrestrial, freshwater, and marine reptiles that have a toothless horny beak and a shell of bony dermal plates usually covered with horny shields enclosing the trunk and into which the head, limbs, and tail usually may be withdrawn.

The naming of the ‘showturtle’ command may be a Logo reference https://people.eecs.berkeley.edu/~bh/docs/html/usermanual_6.... . It’s highly likely that Carmack encountered Apple Logo on the Apple II at some point. Someone should ask him …

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