I am working on this concept for one my groups to explain how monsters/aliens/monsterfied ponies could come to Equestria.

While I was thinking of using some sort of interdimensional portal, I don't want it to be too cliche.

So I thought I would try to ask this question here and see if any of you had any suggestions.

6469228
Perhaps the mirror from EQG? I have a headcanon that the mirror was an interdimensional transporter that could transport you to anywhere throughout space (including other dimensions) but was broken millennia ago and (provided it gets enough power) can only transport you from the pony world to the EQG world.

6469228
Or, if you've played some Half-Life/Black Mesa you could initiate a Resonance Cascade and cause random portals to and from this other dimension (Although I personally like my EQG idea better)

6469236 I had thought originally that the portal might be created by some sort of mutation in the plunderseeds and that when the vines returned to the Everfree, that those that had formed the portal remained.

Sorry I'm not a huge EQG fan.

6469253
Me neither, the only part of EQG I find interesting is the portal its self.