Uhhm Ursula helped them

Really? It's once mentioned in passing how the merfolk joined the previous war between gods and devils. It said how they not only fight the devils and undead on sea, but also on land. For a long established "fact" such as that, we could safely assume they have some kind of potion to make some merfolk able to walk on land. Or they would be able to change their appearance to be humanoid just like the dragons if they're born of some level of merfolk or if they reached some level of power.