Answer:
At first I was going to say my favorite tropical rainforest animal is the toucan, but on second thought, my favorite is the tapir.
Step-by-step explanation:
We must fill in the blank with a preposition that completes the phrase used. In this case, we are using a fixed expression: "on second thought." Thus, no other preposition would be possible here, since the phase is always "on second thought."
"On second thought" is used to indicate that we have changed our opinion or decision about something after thinking about it again. For example: "She was going to order some chocolate ice cream. But, on second thought, she went for the mint one."