Why are most people on this thread talking about war.
Sure there will be cross cultural issues but these kinks can be worked out
The human race is getting better at controlling there destructive side Columbus was around in the 13th and 14 centuries and he behaved like crap toward the inhabitants though they were extremely peaceful we have gotten better since then least we ain't usin atom bombs anymore and even if we did we would only use them in situations that have no other solution
I see it going similar to this
If a pony arrives on earth he/she would either be 1 be attacked by hillbillies and forced into the ky derby
Or 2 treated well and taught about our life and culture heck we may even advance there technology for them
If a human found him/herself in equistria the ponies would probably be a bit cautious at first but will warm up to him/her so long as the human doesn't do anything stupid and again would be taught about life and culture
And before long we have a form of transport between the 2 worlds and we're getting to know each other well