The name does not need to be the name of the lowest level part. See the items that I currently have. There is a lvl 1 part but the name is Aftermoth (a lvl 2 boss). I've seen this before when I had a second lvl 1 part.
I suspect the choice is made based upon the % of the boss that is created. E.g. 0-100% or 0-150% is lvl 1 100-200% or 150-250% is lvl 2 > 200 or 250 is lvl 3.
Lab Pairs 71m, 50f (25%)
Fur Megahurtz (lvl 3) [x]
Horns Shamaniac (lvl 3) [x]
Ear Thug-of-war (lvl 3) [x]
Eye Ark Enemy (lvl 3) [x]
Heart Obscentinel (lvl 3) [x]
Rib Magnum Octopus (lvl 2) [x]
Fillet Ducktator (lvl 3) [x]
Paw Aftermoth (lvl 2) [x]
Hoof Oxydjinn (lvl 3) [x]
Tail Mini-Thug-of-war (lvl 1) [x]
Aftermoth with power of 170% (out of 300%) can be created from these parts.
And apologies for the lazy layout, didn't know how to attach a screenshot