There were several bridges in Ponyville under which Sweetie Belle's socks might be found. Right now however, she was not under the bridge downtown. Rather, she was looking under the bridge that led to Fluttershy's cottage. She found a small creek and a family of otters, but no socks. She tried to figure out how she could make a pun about looking under the "otter bridge", but realized she wasn't really a large enough fan of bad puns for it to be worth the effort. So she went to hang out with the crusaders instead.

Author's Notes:

Why, I crossed that bridge just the otter day!