Happy Fibonacci Day!

J, 58 bytes

3 :''' i_''{~|.|:(,|.)(#&1,2#~0&=)"*}.,0,.(%-.-*:)t.1+i.y'

Uses (%-.-*:)t. for Fibonacci generation. Explanation might come a bit later.

Usage:

   f=.3 :''' i_''{~|.|:(,|.)(#&1,2#~0&=)"*}.,0,.(%-.-*:)t.1+i.y'
   f 5
        ii        
        ii        
      i ii i      
    i i ii i i    
i_i_i_i_ii_i_i_i_i

Try it online here.

CJam, 41 39 bytes

"i""_"1$ri({1$1$4$+}*]),f{Se[}W<_W%+zN*

This prints a fair amount of leading whitespace. Try it online in the CJam interpreter.

How it works

"i""_"1$ e# Push "i", "_", and a copy of "i".
ri(      e# Read an integer from STDIN and subtract 1.
{        e# Do that many times:
  1$     e#   Copy the last underscore.
  1$4$+  e#   Copy the last strings of i's and concatenate them.
}*       e#
]),      e# Wrap the results in an array, pop the last string, and get its length.
f{       e# For each remaining string, push the string and the length; then:
  Se[    e#   Pad the string to that length by prepending spaces.
}
W<       e# Remove the last string (underscore).
         e# We have now generated the columns of the left half of the output.
_W%+     e# Append a reversed copy (columns of right half).
z        e# Transpose rows with columns.
N*       e# Separate the rows by linefeeds.

TeaScript, 93 84 76 + 1 = 77 bytes

+1 byte for "Inputs are numbers?" checkbox

r×ß(p.R((w=F(x©-F(i¬©+"i"R(F(i¬±)t¡ß(j=i<w-1?" ":"_",(A=l¿i>0?j+l:l)µ)+Av©j§

Ungolfed version:

r(x)m(#(p.R((w=F(x))-F(i+1))+"i"R(F(i+1))))t()m(#(j=i<w-1?" ":"_",(A=ls``.m(#i>0?j+l:l)j``)+Av))j`
`

Thanks to @Vɪʜᴀɴ for the tips.