We know that Newt has a Nundu:
The camera pans up to reveal another magnificent creature, the Nundu - looking almost exactly like a lion, it has a large mane that bursts forth when it roars.
- Fantastic Beasts and Where to Find Them (The Original Screenplay)
But we also know that Nundus are extremely hard to catch:
This East African beast is arguably the most dangerous in the world. A gigantic leopard that moves silently despite its size and whose breath causes disease virulent enough to eliminate entire villages, it has never yet been subdued by fewer than a hundred skilled wizards working together
- Fantastic Beasts and Where to Find Them
So how did Newt manage to catch one? He usually doesn't work with others...