Translation:There was a bear with six legs on that beach.
So, is it in the dual, even though there are six of them? Are all the many beaches on the coasts of Bohemia populated by six-legged bears?
no, it was miami beach, bear at 1:13. what is czech/bohemian here is the cultural reference.
czech dual is pretty messy. here we have medvěd s šesti nohama, but it would be pták se dvěma hlavami and stůl se dvěma nohami, pes se třema nohama, and muž se čtyřma očima.
the situation in the plural instrumental is that demonstratives, numerals 3/4, and adjectives agree with the noun, numeral 2 and forms of "oba" (both) are always dual, and the noun is dual if it is one of the four listed dual body part nouns (nohama, rukama, očima, ušima) if used in the body part sense. (should you wonder: prsa, kolena, ramena, stehna, lokty, rty, none of those are so listed despite typically coming two per person.)
another complication is that the dual instrumental endings, under pressure from their much wider use in non-standard "common" czech, feel wrong to many speakers, who overcompensate and incorrectly use the non-dual endings in an effort to sound proper.
Yes, it is not real the dual gramatical number, just a relict, so it is no longer important if they are two or more, just that they are pair organs so they kept the form of the past dual in their actual plural.