word_list = ["cat", "dog", "rabbit"]
letter_set = set()

for a_word in word_list:
    for a_letter in a_word:
        letter_set.add(a_letter)

print(letter_set)

set을 이용해 보시면 어떨까요?

word_list = ["cat", "dog", "rabbit"]
letter_list = []

for a_word in word_list:
    for a_letter in a_word:
        if a_letter not in letter_list:
            letter_list.append(a_letter)

print(letter_list)