I have a PC who is a Necromancer/Barbarian Path of the Ancestral Guardian. The party found some magical items that included things made with dragon scale's and talons. The Necrobarbarian wanted some of these items, not to use, but to disassemble. Obviously, the party opted to not give him the items. Curious though, I messaged him privately and asked him why. His response...he wants to make a Frankenstein of an undead creature using a combination of different body parts. I am assuming that he would use the animate dead spell in order to do this. As the spell reads, I don't see a problem with him doing it-although I would suggest to him that he not disassemble weapons, instead just start collecting body parts. What do you guys think? 

Animate dead applies to a single, whole dead body, not a collection of parts, as I read it. That said, if the end creature just used the zombie stats, and you let it look like however the player wants for purely flavor, I don’t think there would be a problem. 

What it seems the player is actually trying to create, even if they might not know it, is a flesh golem. That I would not allow until very high levels. 

I did read animate dead the same way (needed a skeleton or dead body). There is actually a magic item, Manuel of Golems, that allows for the creation of a flesh golem. That would be the only way I would allow that. Otherwise, I would just go with the whole Zombie stat block (purely for flavor).