Which is the correct glyph origin of 入
http://www.yellowbridge.com/chinese/character-etymology.php?zi=%E5%85%A5
  
  (象形) Pictographic. Picture of the sharp point of an arrow used to pierce something.


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  http://www.chineseetymology.org/CharacterEtymology.aspx?characterInput=%E5%85%A5
  
  Primitive pictograph 入. An open tent door. Meaning enter.


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  http://www.vividict.com/wordinfo.aspx?id=3197
  
  字源解说
  
  入，甲骨文像盒盖向下，表示收存物品，加盖封藏。造字本义：收存物品，加盖封藏。
  
  (入 in oracle looked like a lid facing down to represent "to store items in" , or 'seal something with a lid' .  "Enter" was an extended meaning.
  
  The original meaning of the word: the storage of items, stamped sealed.)
  
  金文承续甲骨文字形。有的金文形似屋“宀”。
  
  (金文 inherited Oracle shape. Some 金文 of 入 looked like "house")
  
  that may had been the origin of 'An open tent door' etymology


After studied the references, my conclusion is the earliest pictographic of 入 resembled a lid facing down.  Then evolved into 'open tent' shape. 

The 'arrow' representing 'enter' (入) etymology is likely an later  interpretation. (depend on how early 'arrow head' was invented, it might had been developed concurrently with 'lid'  for the meaning of 入)