This is the only corruption that I know of at present that is one hundred percent guaranteed fixable in a .petfile or a .baby file. It seems that the bitmap which is at the very end of the file is not included in the checksum, so that if it's missing or damaged you can repair the pet or baby. Don't get too excited though; I cannot guarnatee to fix corruptions in other sections, usually the only thing I can suggest for those is to "clone" the baby or pet.
So, to fix up your bitmap-damaged pet, go down to the last instance of PFMAGICPETZIII. It's directly below the family tree info. In Petz 3 and 4 it'll look like this:
In Petz 5 it might look like the above or it might look like this:
For a baby, go down to the last instance of pfmagic -- it'll probably be pfmagic2, and will be directly below the adopter's name.
Select from directly after the PFMAGICPETZIII or pfmagic2 to the end of the pet or baby. Now open a non-corrupt pet or baby and select from the start of the bitmap to the end of the file in that also. Choose "copy" and then "Paste" it over the selected part of the damaged pet or baby.
Open the game; ta-da! The afflicted one will be corrupted no more.