Precise scientific use began to develop mid-18c., but the word was used as well at first of animals now known as amphibians, including toads, frogs, salamanders; separation of Reptilia (1835 as a distinct class) and Amphibia took place early 19c.; popular use lagged, and reptile still was used late 18c. with sense "An animal that creeps upon many feet" [Johnson, who calls the scorpion a reptile], sometimes excluding serpents.

And the terrestrial animals may be divided into quadrupeds or beasts, reptiles, which have many feet, and serpents, which have no feet at all. [Locke, "Elements of Natural Philosophy," 1689]

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An inadvertent step may crush the snail
That crawls at ev'ning in the public path ;
But he that has humanity, forewarn'd,
Will tread aside, and let the reptile live.
[Cowper, "The Task," 1785]

The Old English word for "reptile" was slincend, related to slink.