The word frog was used as as a general term of abuse in Middle English, and was applied specifically to the Dutch in the 17th century; its application to the French (late 18th century) is partly alliterative, partly from the reputation of the French for eating frogs' legs.
Allusions are also found to a traditional fairy story, recorded by the Grimm brothers, in which a frog in a pool returns a princess's lost golden ball in return for her promise that he may live with and be loved by her. When he claims the reward her father makes her keep her promise; the frog eats from her plate and sleeps in her room. In the original story it is when she has thrown him against the wall that he turns into his real shape, that of a handsome prince, who is now her lover and husband; the usual version is that it is when she kisses him that the enchantment is broken and he is restored.
have a frog in one's throat lose one's voice or find it hard to speak because of hoarseness. The expression dates from the early 20th century, but frog here relates to an earlier meaning of a soreness or swelling in the mouth or throat.