The link for the pattern can be found here.
I only made modifications to the 'leg'. I didn't want to crochet onto the metroid's body, so I worked the pattern backwards. I also stopped at the third round to make the 'legs' shorter.
My modifications to make the leg are:
Rnd 1: 3 sc in magic ring (3)
Rnd 2: increase around (6)
Rnd 3: 3 sc, 3 dc (6)
F/O and leave tail to sew to body.
I then wanted an even bigger metroid. To do this I used 3 strands of yarn together and a K hook and followed the pattern with my modifications. The big metroid is almost 5 inches tall.