Hi, I checked on wiktionary and the masculine form in nominative case is erwachsener and the feminine erwachsene, plurals for both also being erwachsene. However, on DL, the male form is erwachsene as well. Welches ist richtig ??
Erwachsener is a nominalized adjective, which means that it follows the declension of an adjective, so you say "ein Erwachsener" but "der Erwachsene". The default nominative form is the strong ending -er on wiktionary, but apparently duolingo chose the weak nominative ending -e.
In German, you have the adjective erwachsen which means adult:
Ein erwachsener Hund = an adult dog
Eine erwachsene Person = an adult person
This adjective may also be used without a noun, in which case it is used as a noun on its own, meaning "adult". It is capitalised and is grammatically a noun, but still declines like an adjective (but it's not an adjective !):
Ein Erwachsener = an adult
That's because the declension of German adjectives is quite complex, and adjectives may take different endings depending on the article before it.
I don't really have time to explain it all, but the basic idea as that
"Der erwachsener Hund" would sound redundant because the r of der is present twice, so we remove it and we get "Der erwachsene Hund"
While in "Ein erwachsener Hund", there is no r on ein so the r of the adjective can be kept without sounding redundant.
Of course it's a bit more complex than that, but this gives you a little idea of what it's all about.