Bigpapi4153

Hi there, I could use some assistance. I just logged onto my account in the Hoenn Mauville Pokecenter and noticed that my starter (Marshtomp- I believe level 26) has been replaced in my party by a level 5 Onix!!! I have no idea how this would have happened and I 100% did not release my Marshtomp (I don't even know if that is possible for your starter Pokemon). Help with this would be much appreciated!! P.s. This was on the silver server and my username is Bigpapi4153