>>23771325to explain my example: Skorpions appear to have 10 legs (8 legs + claws) but actually the claws (pedipalpes) are modified antennae.
another funny example is the vinegaroon which appears to have six legs + claws.
like with the scorpion the front legs/claws are modified antennae, interestingly they are followed by antennae, which are actually modified legs. So he has legs that actually are antennae and antennae that are actually legs.
tl;dr: limbs dont appear out of nothing