Actually, that's pretty cool. There are a few IRL instances where parasitic organisms control the behavior of its host (though not to this degree). I didn't know that Pokemon had a few of these cool but more disturbing bits of biology (with the exception of Cubone)

Let's not forget Slowking goes from pea-brained to supergenius upon evolution because the Shellder latched onto its head is always biting its fangs into its skin and excreting venom into the bloodstream, pumping its brain with a constant supplyof POISON. That's right- Slowking is being drugged with poison. So yeah, that Pokemon acts dopey 'cause he's high as f#ck.