Is the long ending real or not? The short ending (where the gospel ends at 16:8), is in two 4th century manuscripts, one 12th, and the Syriac translation. The intermediate ending has one extra verse, but it only in one manuscript. Every other manuscript that has this extra verse also includes the long ending! The long ending (which has verses 9-20) is contained in all other 1600+ manuscripts. There is actually an even longer ending in one manuscript. While some church fathers/theologians leave out the longer ending or don’t seem to be aware of it, two of the earliest church fathers quote it (Justin Martyr and Irenaeus; both 2nd century).
Is there another reason the long ending could have been left out? It could have been left out by accident. One of the two early manuscripts that leaves it out may actually leave room for it. It could have been left out deliberately. The ending of Mark was removed from the other early manuscript and different sheets were included that omit the long ending. It could have been left out because of liturgical reasons (Mark 15:43-Mark 16:8 was read for the 3rd Sunday after Easter). For all practical purposes, the gospel ends when the liturgy moves on.
Some also point out that the ending has a different style and vocabulary from the rest of the gospel. However, if you consider that different events/settings will demand a different vocabulary, this is very easily explained. For example, Mark 14:42-52 has 15 words that are completely unique to Mark! There are also various thematic elements in the gospel that continue in the ending.
My view? The long ending was known about at the earliest times, could have very easily been left out either accidently or deliberately, and is not drastically different from any other section of Mark in either vocabulary or theme. The long ending is original.