Why did the chicken wear a snorkel?

According to the Hay Day wikia it was to celebrate their birthday:

On July 10, 2013 (and the following year), to celebrate Hay Day's birthday, all animals were updated with new summer clothes and appearances. For the chicken, it's updated appearance includes a blue snorkel mask. On Oct 16, 2013, the chickens had carved, smiling pumpkins attached to their bodies for Halloween.

It is also in the update logs(v1.11.47):

• Summer Theme: Your farm animals love the summer! How do you like their fun and exciting new looks? All your favorite farm animals in sun glasses, snorkels and Hawaiian shirts!

In the next update, it was removed:

End of summer:

• Reverted the summer clothing from the farm animals
• Removed the exclusive summer decorations from the shop
• Reverted the happy birthday loading screen back to normal

So you will not be able to bring it back, as it was a game update.