Did you know that Lewis and Short's dictionary doesn't contain the word "gullible"? That is an appropriate question for the first of April, but it turns out that I indeed found no trace of that word. Perhaps I'm missing English synonyms for someone who is naive and easily fooled. What would be a good classical Latin word, noun or adjective, for this kind of a person?
credula nec ravos timeant armenta leones
gullible cattle no longer fear tawny lions
Horace, Epodes, 16.33
Credula si fueris, aliae tua gaudia carpent
If you are gullible, other women will seize your joys
Ovid, Ars Amatoria, 3.661
Haec enim etiam in fabulis stultissima persona est improvidorum et credulorum senum.
For even on the stage the silliest characters are improvident and gullible old men.
Cicero, De Amicitia, 26.100
The English verb 'gull' means 'deceive', and 'gullible' is derived from it. Penelope's credulus is excellent (and, indeed, is the word that I would choose), but I should like to point out decipiendus as an apt alternative.