Looks like a spring peeper, maybe my eyes fool me and I am not seeing an X on its back. If it does/did have an X. then it was/is a spring peeper, Psuedacris crucifer.
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"We must not look at goblin men,
We must not buy their fruits:
Who knows upon what soil they fed
Their hungry thirsty roots?"
but be carefull... if the X is to big... sort of an X, and your visitor comes and go, and sometimes is difficult to see, then you should call Mulder and Scully!!!
I know, I know, silly joke... but it was to easy to let it go without a joke :P