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A fan recently asked PLL writer Andy Reaser this very question, saying, "Is it safe to assume that every time we have seen the baby monster costume that it was Garrett?"
Andy replied, "it is not safe to assume ANYTHING in Rosewood." Anyone who had assumed Toby (Keegan Allen) could never be part of the "A" team (aka, most fans!) knows how true that is.
As we pointed out above, we already knew that not every person in a baby monster costume could be Garrett, since there was more than one person in the costume in "The First Secret." But does Andy's tweet also mean there could have been more than one person wearing it during "The Is a Dark Ride"? What about when someone wearing the costume talked to Lucas (Brendan Robinson) during the PLL webseries, Pretty Dirty Secrets?
All we know is we're going to keep our minds open to alternative theories.