RuneScape Classic Wiki
The main RuneScape Wiki has an article on: Toad legs
The Old School RuneScape Wiki also has an article on Toad legs.

Toad legs are used as an ingredient in gnome cooking, and edible by themselves to heal 3 Hits. Toad's legs are obtained by removing the legs from a swamp toad, found at various swamps around RuneScape Classic.

Toad's legs are used in gnome cooking to make tangled toads legs, toad battas, and toad crunchies. This makes the toads west of the Grand Tree especially useful because of their proximity to Hudo Glenfad, who sells the rest of the gnome cooking ingredients.