змея не видит (кого? что) крольчонКА.
accusative detected.
Also I've forgot to draw your attetion to this:
"Крольчонок никогда раньше не видел змей"
There is a plural noun "змеи" in accusative case.
why plural?
becouse he had never seen even one snake before and when we translate this into russian we should say about all snakes as biological species.
"никогда не видел ни одной змеи" (single)
triple negative sounds bad
никогда не видел змей (plural) it sounds much better