The Goat Spawn are already on the board through the Cultists' Reckoning effect. They were set aside at the start of the game as instructed. Secondly, what you're describing sounds like an Ambush effect while this is a Spawn effect.
Yeah, it's only an ambush if it says ambush; this is indeed a (nonstandard) spawn effect and can thus fizzle if the token is in use (for ambushes you can always look at the token wherever it may be so those do not fizzle).
You can do a lot worse than merely getting a wasted encounter in EH.
(I guess one could move the nearest appropriate monster to the space in question if one wants to avoid this waste for thematic reasons, much like new ship spawns in M&M move the old version of that nation's ship)