This post at Mental Floss stated that a group of tortoises is called a 'Creep'. I had never heard of that before, and in trying to find the references for the name, I was surprised that I couldn't actually find anything.
Wikipedia showed no results in either of their lists for animal names.
What I found was that creep is a collective noun. The professor Peter Trudgill uses the word in a chapter about collective nouns and the example is of tortoises. I don't know what book to tell you to look in though. He is professor of sociolinguistics. From a search on collective nouns for animals, turtles, and reptiles, I only found turtles having the following collective nouns: bale, nest, turn, dole.
My guess is then that Peter Trudgill made up the word and some people have adopted it or it is primarily an English (England) word.
I have worked in various parts of the world in sales of engineering equipment, but have always been involved in animal conservation, especially for turtles and tortoises.
One of my previous employers, now an engineering company was originally purveyor of turtle meat to HM Queen Victoria. They provided the Green Turtle Soup entrée for Queen Victoria's Coronation Banquet, and for the Royal Family in general up to H.M. Queen Elizabeth II, who also had green turtle soup at her coronation banquet. It was not on ration points. The company had the royal warrant.
I'm afraid the general terms for tortoise and turtle in English come from the meat trade.
A "creep" originated in the Galapagos, where the tortoises were hunted often to extinction, by sailing ships crews, whaling or other, and even HMS Beagle.
Once on board a creep became a bale, also the usual description for sea turtles caught by nets, and stored, like their land cousins in tightly bound nets. There was a number of individuals and weight for a bale. There was a market standard for different species - say 4 Leatherbacks per bale, 6 Green per bale. Green turtles were favoured as they had tasty meat but did not expire quickly.
If the animals were caught by bill hook or harpoon they were put in a 'drag bale', thus injured they would be the first consumed by the crew. Those captured at sea or on land put in nets were "bales" (uninjured) when landed. Galapagos Tortoise may be one per bale.
There were markets all over the world; in London, it was Billingsgate for the meat, and for the shell it was Lower Thames Street. Turtles were delivered live to restaurants. Generally, the voyage and the occasional wash with water had purged them. Shell and meat would then be subdivided with other terminology, depending on the quality of species meat and shell.
Obviously, there are many other terms for chelonian meat and shell in other languages - florid descriptions in China and French/Creole on the East Coast of America for the Diamond Back Terrapin and the alligator and common snapping turtle, for instance.
I obtained this information from an old employee of the former employer's green turtle soup restaurant at a company dinner in London. No, we did not have turtle soup, and I did not kill him. He was a really nice guy and a war hero with the London Scottish Regiment. For the shell business, I know something from another of my employers. We were involved in trading heavy equipment to the former Soviet Union. Our offices were next to the dreadful fur trade warehouses on Lower Thames Street, which was next to the Turtle Shell Exchange (this may not be the correct title of the market).
My point is that our naturalist forbears took collective nouns, in this case, from general parlance. They were generally scientists. Conservationists came later. I think the collective noun was thus applied outside the tortoise/turtle trade to all species by English speakers.