"Frogs' legs" and "frog legs" are both correct, but "Frogs' legs" is the most common.
"Frogs" is a plural noun. When we talk about their legs, we need a possessive. When a plural noun ends with "s," we only add an apostrophe: "Frogs' legs"
A lot of my students asked me about this English skill, so I made a video about it:
I hope that it's helpful.
"Frog legs" is also correct. We use "Frog" like an adjective, but it's a not an adjective. It's a noun. We call it a noun modifier.
"Frog's legs" is incorrect. "Frog" is a singular noun. I could say "A frog's legs" (It means "One frog's legs."). That is correct. However, we usually don't say "I ate one frog's legs last night." A little bit strange, right? Instead, we say "I ate frogs' legs last night." I'm telling you a story about the kind of food that I ate (general), not about one poor frog!