Looks like Fluttershy's brother Zephyr was the one wriggling around in her toes. He explained to her how he wanted to get a much closer look of the girls at the beach by shrinking himself down so he asked Zecora for a shrinking potion in exchange for a free hair service by him. After he used it he didn't exactly end up in the spot he wanted to be in to look at the girls, nor did he fully read the instructions. It came with a side effect which randomly teleports the user to a random area only once and he ended up between his sister's toes instead of between Rarity’s chest or somewhere where he can view them. After hearing this confession Fluttershy scolded him and made him stay in between her toes for a while during her relax time. Well, at least he can view his sister for a while lol.