Hi Araylie,
The last boss should drop at least one new part that you haven't discovered before. If it didn't, it might be a bug - I'll take a look if that's not the case.

There currently are 5 breeds with 4 parts each and 11 breedless parts.

Hi there,
so I tested what you told me, and you were correct.

https://imgur.com/a/UEc1opO

I see what the game did.
From the screenshot above, the game gave me "Twister" and "Xenoplate" because I didn't have them in the Aquarium mode. However, those items are not new to me since I already unlocked the breeds "Advena", "Cra'Than", "Buginis" and "Saenus Beefuorus".

I may suggest for boss to offer at least one breedless part the player didn't unlock yet.

Oh... Huh...
After I took the screenshot of the "Twister" and the "Xenoplate",
I immediately exited the game to verify what I had unlocked in my Aquarium mode before grapping anything...

Then I return and continue my Load Campaign, but now the rewards are gone...
And in the end, I didn't unlock anything.

I must say... Few breedless parts need to be stronger on their own...
The swarmer firerate / respawn of the critters is too slow...
All the critters die by other projectiles before reaching the target and they barely deal any damage even in mass.