See my post just above yours. In short, you have extra files in your PROTO/CRITTERS folder. Delete the files 00000XYZ.pro, where XYZ is one of 062, 079, 089, 097, 107, 136. 152, 160, 161, 163, 164, 342, 471, 546, 550, 569, 585, 643, 656, 658, 659 or 660. Actually, 585 may be necessary, If you do a certain procedure with Lenny and encounter problems. I don't have quick access to that part of the game. These files may come back if you do any modding of your own, so the process might have to be repeated at some point. I don't know why they are included in the Megamod, or whether there's a better way of putting modified base NPC protos in the mod.