I recently came into ownership of a hellhound who is only 29. I don't have the best communication with her yet and I was wondering, because of their long lifespan, if that could make her a puppy.
I'm soon going to get another hellhound and thinking about it recently if I ended up getting a male is there a chance they could mate (of their own free will, I'm happy with just two).

I know very little about the lifespan of hellhounds, so unfortunately I can't answer your first question. As for the second question, mating depends on if the possibility exists. If they are both living entities and not spirits, there's a chance they could mate. But if one or both are spirits, it isn't possible, as the dead cannot procreate.

Mmm try asking the conjurer about it.

I swear someone asked this same exact question on this forum not long ago.