Recently, the tag "flutterbat" was aliased to fluttershy_(mlp) because some users and admins found the name to be unnecessary, since the character itself is a "bat pony" version of Fluttershy, which means that you should be able to find roughly the same content if you search for "fluttershy" and something like "bat" or "bat_wings".
However, I think this needs to be revisited, given the fact that they actually refer to Fluttershy as "Flutterbat" in the show itself, making the name canon for Fluttershy's "bat pony" form. Flutterbat's appearance in the show is also significantly different from Fluttershy's, so there should be no confusion regarding when Flutterbat should be tagged and when Fluttershy should be tagged.
Given this information, I think the tagging of Flutterbat should work just the same as how e621 currently handles the tagging of characters in costumes (which is also up for review here: https://e621.net/forum/show/98600), meaning that any posts with "flutterbat" would also be tagged with "fluttershy" as well.
Please remember to keep your responses civil and constructive. Thanks!
Updated by 123easy