CARNAC (the function that interprets file names of imported files) will remove "trailer" from the name in the [Name] tag if it correctly identifies the movie. You can't stop that, but you can set the [Name] tag to whatever you want.
There are a huge number of ways to handle movie trailers in MC. A search of the forum will give you lots of ideas. It seems you have decided to import them like movies and have them show in the movie view, with just the [Name] tag differentiating them. Honestly, using that approach, if you aren't importing many movies the easiest way to fix it is just to edit the [Name] tag and add "-trailer" back in.
But you could tag videos on import, with an expression that recognises when a trailer is being imported by checking the file name for the text "trailer", and then writes the [Name] tag as you want, but only for movie trailers.
While you are doing tagging on import, you could also set the [Media Sub Type] to "Trailer", which is one of the options, and then modify you movie views in Standard and Theatre View to include trailers. Or you could set up a separate Trailers View.
A bit of research and learning how to write expressions, and you should have the solution you want.