Morphing Words

Find a pair of words of equal length, a primary word and a secondary word,
such that each letter of the secondary word can substitute for the corresponding letter location in the primary word to make a new word.

Example:

Primary = dog , and secondary = fit . f substitutes for d to give fog , i
substitutes for o to give dig , and t substitutes for g to give dot .

Here are two examples of this puzzle, the first harder than the second.

1)  Find a mammal (primary word) and fish (secondary word) both in 4 letters.

2) Find a female animal (primary) and offspring of the same species (secondary)
both in 4 letters.

Main List of Word Puzzles