2 year project for a new game
That certainly depends on the skills you and your team members have. How is your team structure? And how many will work full time in the two years?
The main problem that most game engines won't solve is the destructible environment. Also does breaking the world mean that they can simply break down walls, or does it mean full Minecraft?
Second I don't know how many engines that have a dynamic world loader, but that is something you should be able to add to most engines never the less.
Sorry for the short answer, but I can't tell if you are a beginner in way too deep water, or if you represent an established team.
I read the fan fiction book "Fallout Equestria" and decided that it deserved a game.
You shouldn't proceed without permission of the author. It complicates things even more that the thing you want to base your game on is fan fiction (which, if you're using the term "fan fiction" correctly, means the book itself is a derivative of someone else's IP, so the fan fiction author isn't even the overall owner of the underlying IP - see the Business/Law forum for more on that, or if you want to post questions about IP ownership and permissions).
Subject: 2 year project for a new game
...the size of it may end up becoming larger than the world of skyrim... should i build my own engine or should i use engine of another game...
People will come along and give you advice about engines (and tell you that you should read this forum's FAQs), and tell you how to go about starting your own game. Clearly, you're biting off more than you can chew, but I don't expect you to be dissuaded by my saying so.
I would love the help of others for this
You can use the Classifieds section to seek team members (recruiting is not permitted in these discussion forums). If you ever have questions about managing a project, ask those in the Production/Management forum.
Have a look at Unity 3D or Unreal Engine 4. Sounds like you trying to something quite Highend (3D and good graphics I guess), so forget about building your own engine. You could sink 5-10 years just into building that engine if you want to do it right (might be higher or lower, still, will take a LOT of time for a highend 3D engine).
An existing Engine will let you start developing your game sooner than when you first have to do the low level plumbing.
Be aware though, no existing engine will free you from the need to both be quite versed in programming (given you don't find someone who does the programming for you) or Graphics (given you don't find an artist).
Then, about those selfless helpers. Try the classified, maybe you find people that like to help. Most probably not, as long as you cannot pay them. If you can't, and still would like to get help, you need to have a VERY GOOD pitch to draw people in to help. So either start showing your skill in art by creating concept art, or start building a prototype with placeholder art.
Both will show potential helpers that a) you are not an idea guy not doing anything to the project (yes, the idea alone does not count for most people), b) you are already on the way and have one area covered (to some extent), and b) yes, you are serious about the project, and will not drop it in 3 months when you lost interest.
next, your 2 years. Forget that. From your vague ideas this project sounds more like 10 years for 1-5 people to work on (part time, I guess). You can shrink the time needed tremendously by making either compromises (smaller scope, less game design elements, simpler graphics), or working smarter by employing procedural technologies (though this you will have to develop on your own mostly, and then use that in a clever way).
Really think about the scope again. Does it need to be 3D? Does it need animated characters? Does it need destructable terrain? Does it need the huge scope?
Pick the things that really NEEDto be in the game.... prune all others back to the minimum (you need large outside levels.... but it can be in a very simple 3D graphics style, without animations, and you only do a very simple game design. Most of the level geamotry is created procedural, even though that might mean the average quality of the level design takes a hit because it lacks the human refining)
