I always imagined these were like, the size of commercial jets

Not a beast

My party managed to steal a baby roc from his mother and after an hour (irl) they lost it and left it to destroy sparta. We are currently level 7. Now they want to tame it and use it as transportation. They also made a nemesis. This all went down as we listened to the freebird solo on loop. 

GOD I LOVE DND!

rocs fall, roll iniative

haha!!!!!!!! good one.

can i tame one?/can i have it as a pet?

(we found and beat one and now it is on 10 hp)

Me and two other 5th level characters took down a roc!

How did the Aboleth get in the lake?  Roc dropped it!

Mark of Handling Human

I wish familiars could attack now. Dis thing is op.

I don’t believe that this thing could take an adult dragon.

My party tod me to "Make things interesting" after half a session of domestic nonsense. two of them are dead and we're resuming combat next session.

Most DMs like to grapple fly up and then drop the poor soul they snagged. If someone doesn’t have featherfall that’s a lot of damage.

 big bird

Fairly sure it is actually possible since they have three int. Mark of handling human allows you to cast animal friendship on monsters. Pretty exciting.

This is an awesome creature! As badly as I want to send this information to my DM, I am absolutely terrified of what they will do with it :,D

So how to make one of these into interesting battle?

They don't have much going for them, the multi-attack with auto grapple on talons attack is quite clear sign on how they fight, approach quickly (dive with dash to 60ft distance, then you can get as far after attack on next turn) try to pick someone up, and fly away. It can quite quickly get high enough that even if rest of party shooting after it and the grappled character manage to kill the roc, the grappled character is likely to die from fall.

Roc's size and fly speed means it is also quite difficult to track positions on map. If the party was on middle of my battlemap, a roc would spend at most 3 turns on it, and the more important factor in positions would be height, as it can hardly take cover. But for that time it would cover half the map with it wings. Against dashing draft horse, Roc gets 40 ft per round without dashing by flying forward, so that would keep it in longbow range for long enough, maybe even on melee range for some time.

Quote from ESTRELARIUS >>

Funny Thing, it can be defeated  by a 1th level spell (Beast bond).

That is all kinds of wrong.  First, a Roc is a monstrosity, not a beast (only creatures that have the "beast" creature type are actually beasts in 5e).  Secondly, if it were already hostile to you, that also disqualifies it from Beast Bond.  And the spell says nothing about forcing the creature to act a certain way, so it wouldn't be a means to defeat it even if you could cast it on a roc.

Rocs and Tarrasques have a lot in common. They’re both big, useless sacks of hit points.