Cthulhu

Greg_Deckler, here's an approach using the newer WINDOW functions.

Local Group Index  = 
VAR _GlobalGroup_ = /* All rows with current animal up to current row */  
    WINDOW (
        1, ABS, /* From frirst row */
        0, REL, /* Up to current row */
        ORDERBY ( Cthulhu[Index] ),
        PARTITIONBY ( Cthulhu[Animal] )
    )  
VAR _AddPrevIndex_ = /* Lookup previous index for each row */
    ADDCOLUMNS (
        _GlobalGroup_,
        "@PrevIndex",
	        VAR _PrevRow_ =
                OFFSET ( -1, _GlobalGroup_, ORDERBY ( Cthulhu[Index] ) )
	        VAR _PrevIndex =
                MAXX ( _PrevRow_, Cthulhu[Index] )
	        RETURN
                _PrevIndex
    )
VAR _Skips_ = /* All rows where prev index isn't current index - 1 */
    FILTER ( _AddPrevIndex_, [@PrevIndex] < Cthulhu[Index] - 1  )
    /* Will be empty if there are no such skips */

VAR _LocalGroupStart = /* Latest skip -- blank if _Skips_ is empty */
    MAXX ( _Skips_, Cthulhu[Index] )

VAR _LocalGroup_ = /* All rows since latest skip */
    FILTER ( _AddPrevIndex_, Cthulhu[Index] >= _LocalGroupStart )

VAR _Rank = /* Current row is the last in the local group */
    COUNTROWS ( _LocalGroup_ )

RETURN  
    _Rank