Reincarnation in AD&D is the magic-user version. It is a 6th level spell that allows the caster to bring the dead back, though rarely in their original form. I have enjoyed using it and Reincarnate (the druid version that turns the dead into animals) for many years but like most things, there is room for improvement. In this case using rare magical monster body parts allow for a chance that the resulting creature isn't humanoid. Personally I would put it at 25% but I see that many people would prefer to put it at 5% or even less because of the possibility of turning a PC into a player controlled monster. Here are three examples that use creatures only from the first edition monster books:

Derek Holland is the author of Mutant Future and Labyrinth Lord material in multiple Skirmisher Publishing LLC publications and co-author of its Wisdom from the Wastelands game supplement.

Reincarnation in AD&D is the magic-user version. It is a 6th level spell that allows the caster to bring the dead back, though rarely in their original form. I have enjoyed using it and Reincarnate (the druid version that turns the dead into animals) for many years but like most things, there is room for improvement. In this case using rare magical monster body parts allow for a chance that the resulting creature isn't humanoid. Personally I would put it at 25% but I see that many people would prefer to put it at 5% or even less because of the possibility of turning a PC into a player controlled monster. Here are three examples that use creatures only from the first edition monster books:

Deva feather- baku, couatl, dragon horse, foo dog, foo lion, hollyphant, lammasu, moon dog, opinicus, pegasus, shedu, unicorn. Most of these are very powerful because the good monsters tend to be such. It wouldn't be difficult to create new, weaker monsters to fill in the secondary chart. Note that the feather has to be donated. If taken off a dead deva, use the hell hound tooth chart.

Hell hound tooth- death dog, devil dog, displacer beast, hell cat, hoar fox, leucrotta, nightmare, peryton, shadow mastiff, yet hound. I tried to stick with canines, but there just wasn't enough of them so I added a few others based on animals.

Gibbering mouther eye- eyekiller, gambado, garbug, grell, kampfult, imorph, manticore, peryton, sussurus, tiger fly, vilstrak, witherstench. In this case I was looking for creatures that appeared to be twisted or warped rather than the classic aberrations (aboleth, beholder, illithid). Oddly there aren't that many creatures in first edition that fit that description so I was limited in my choices. I skipped the tirapheg because it is just too weak and difficult to play and cut a few that just weren't warped enough (drelb, vulchlings, tentamorph).

I was also going to do one based on plants but the great majority of vegetable monsters are not worth considering for a PC. Even those that are- treants, shambling mounds and some of the fungi- are way outside what players want or are too limited in where they can go. Of course it wouldn't be difficult to design a few plants meant specifically for PCs, other characters and monsters to be reincarnated into. Or making any chart that you want.