“Knot” describes exactly what it looks like: snakes tangled together closely enough that individual bodies become hard to tell apart. Unlike some of the vaguer collective nouns on this list, this one has a well-documented real-world counterpart: the mating ball.
In species like the red-sided garter snake, thousands of males emerge from a shared winter den in early spring and converge on the small number of females leaving at the same time. Dozens of males can wrap around a single female simultaneously, forming a writhing, tangled mass that can look, from a distance, like one large moving knot rather than many individual snakes. These mating balls, most famously documented in Manitoba, Canada, are one of the most dramatic group behaviors in the entire snake world.
Outside of mating season, “knot” also applies more generally to snakes overwintering together in dense, tangled dens, for warmth rather than reproduction.
Example: “By the den entrance, a knot of snakes had already begun to form as the temperature dropped.”
Related: Collective Nouns for Reptiles · What Is a Group of Snakes Called?