It's been a long time since third gen came out, and we still haven't found Nobelium's first form. I've written this post to help us eliminate possible mothers for it until we've found what it is. Let's assume final form monsters. This gives us:
Pershire, Cylindare, Spirifly, Forodus, Arabat, Krakrow, Geparnic, Fertail, Alchemandr, Mumboom, Borbeque, Dummallow, Kiricock, Dragoonish, Banquefly, Owledge, Anastork, Mangrodon, Locrubat, Garmoth, Ferdule, Lumenore, Pharaohbrah, Stegaleon, Bichador, Jestray, Lancegar, Zwergnome, Haligarn, Poocheer, Pupom, Demonike, Mucinox, Batnana, Toadxin, Wallorus, Clubrute, Finotaur and Mermeruch.
I'm going to go ahead and eliminate the monsters which are already known mothers of another monster. Also Zwergnome and Jestray because they only exist as males. That leaves us:
Pershire, Forodus, Arabat, Krakrow, Geparnic, Fertail, Alchemandr, Mumboom, Borbeque, Dummallow, Kiricock, Dragoonish, Banquefly, Owledge, Mangrodon, Locrubat, Ferdule, Pharaohbrah, Bichador, Lancegar, Haligarn, Poocheer, Pupom, Demonike, Mucinox, Toadxin, Clubrute and Finotaur.
I'm going to present these monsters as a list so we can rule them out. Please weigh in and edit them, adding the number you have bred. It will probably take a while, but once we can rule a bunch of these out our job will be considerably easier. Remember, it only counts if you bred it as a final form. If the mother was an earlier form, don't count it towards your total breeds. Also, only monsters you've bred after the 3rd-gen update count. If in doubt, don't count it.
Pershire -2 breeds Sandb0xx, 7 breeds Selltheworld
Forodus - 4 breeds Old Bertie, 2 breeds Kevpar12
Arabat - 6 breeds Old Bertie, 4 breeds Kevpar12
Krakrow - 7 breeds Sandb0xx
Geparnic - 8 breeds Old Bertie
Fertail - 6 breeds Old Bertie, 35 breeds Sandb0xx
Alchemandr - 5 breeds Sandb0xx
Mumboom - 6 breeds Kevpar12, 5 breeds Selltheworld
Dummallow - 2 breeds Sandb0xx, 4 breeds Kevpar12
Kiricock - 7 breeds Old Bertie
Dragoonish - 4 breeds Sandb0xx
Banquefly - 2 breeds Sandb0xx
Owledge - 5 breeds Sanb0xx
Mangrodon - 3 breeds Sandb0xx
Ferdule - 10 breeds Selltheworld
Pharaohbrah - 3 breeds Sandb0xx
Bichador - 10 breeds Old Bertie
Haligarn - 11 breeds Old Bertie
Poocheer - 4 breeds Old Bertie
Demonike - 4 breeds Kevpar12
Clubrute - 2 breeds Sandb0xx, 2 breeds Selltheworld
Finotaur - 2 breeds Old Bertie
I've bred a lot of earlier forms as well, but I'm only going to add them if this avenue doesn't lead anywhere.