caught my brother getting his cock licked by our dog. he's sixteen.

normal? i've heard of people doing this.

What's normal mean? My concern would be that dogs use their tongues for very different purposes than we humans do. Who knows what the dog was licking before your brother's cock! That said, I let my cat lick the tip of my cock once. Very rough and not particularly nice.

When we were about 10, my pal led me to a hiding place and his bulldog licked our little dicks until the feeling got so good we pulled back. We hadn't learned to JO. This was the only time, but I would have done it again. It's just ordinary kid stuff in my opinion, normal at 16 or any other age.

I can't say I've ever experienced anything like this. But I would suggest that it is probably a normal process of curiosity.
The problem arises when it moves on from that.

Did you speak to him about it? It might not be a good idea to right away, but if you discover this again it might be worthwhile having "that" conversation, explain the social norms and so on.

It would be difficult to approach without making him feel desperately mortified, so it would have to be done in a very adult way. Most importantly, be his older brother and make sure you maintain trust.

It's unlikely to develop into a socially unacceptable persuasion, but there is always that risk if more things are noticed but ignored.

I hope that makes sense?
It's only my opinions though, so I may be completely wrong.

Imagine yourself in his place; he's not a child, and trying to tell him what he should or should not do is a quick way to fracture a relationship with a teenager.
I suggest: Ignore what you saw, let him find his own way sexually, meantime wipe the memory from your consciousness, and he'll do the same.

Pretty normal for you to catch him.