Ano means anus, while año means year.
One of the first sentences a student of Spanish will learn is "¿cuántos años tienes?", how old are you, literally "how many years do you have?"— often before they really grasp the distinction between ene and eñe.
And indeed I see this on etymonline:
> from Latin nucleus "kernel," from nucula "little nut," diminutive of nux (genitive nucis) "nut,"
So for whatever reason a new word was called for, and our nucleus comes from that, not from the diminutive form of "nut".
I've been wondering about this! Does Spanish regularly form diminutives other than with the -it- infix? ("m'hijito" etc.) Latin forms them with -ill-, -cul-, or -ul-, and many such forms are obvious in Spanish (tomatillo / quesadilla etc.), but I've never known whether -ill- is still productive today or if those are relics from the past.
It's fun to see a Spanish region bordering Portugal (-inho) forming diminutives with -iño and one bordering France (-ette) forming them with -ete.