“Knob” is another entry in the small family of toad-specific collective nouns that also includes “knab” and “knot,” and it’s not hard to guess where the name comes from. Toads are famous for their bumpy, warty-looking skin, dotted with raised glands (the “warts” aren’t actually warts — they’re parotoid glands used for defense), and a “knob” is exactly the word you’d reach for to describe one of those bumps.
Applied to a group, the term likely does double duty: describing both the toads’ texture and the way they pile together into a small, rounded cluster when sheltering. As with several of the more obscure toad collective nouns, a documented historical origin isn’t available, but the descriptive logic lines up cleanly with how the animals actually look and behave.
Toads’ bumpy skin isn’t just cosmetic. Those parotoid glands secrete a mild toxin that discourages predators from taking a bite, one of the reasons toads can afford to be slower and less water-dependent than frogs. A knob of toads, tucked together under cover, is a group that’s built to be left alone.
Example: “A knob of toads sat motionless under the porch light, waiting for insects drawn in by the glow.”
Related: Collective Nouns for Amphibians · What Is a Group of Toads Called?