This is an PLUTO
This is PINEAPPLE
This is ORANGE
This is RICE

PLUTO:
This is an PLUTO
PINEAPPLE:
This is an PINEAPPLE
ORANGE:
This is an ORANGE
RICE:
This is an RICE

$ awk '{ print $NF ":"; print }' file
PLUTO:
This is an PLUTO
PINEAPPLE:
This is PINEAPPLE
ORANGE:
This is ORANGE
RICE:
This is RICE

awk '{ print $NF ":" ORS $0 }' file

awk '{ printf("%s:n%sn", $NF, $0) }' file

$ sed 'h; s/.* //; s/$/:/; G' file
PLUTO:
This is an PLUTO
PINEAPPLE:
This is PINEAPPLE
ORANGE:
This is ORANGE
RICE:
This is RICE

h;          # Put the pattern space (the current line) into tho hold space (general purpose buffer)
s/.* //;    # Remove everything up to the last space character in the pattern space
s/$/:/;     # Add colon at the end
G;          # Append the hold space (original line) with an embedded newline character
            # (implicit print)

awk '{$0=$NF":n"$0}1' file