"The river has crocodiles"
Translation:Mto una mamba
Mto is a class three noun (a singular m-mi noun). The subject prefix for that class is u-.
Mna would mean either (1) "you [plural] have" or (2) "there are ... inside". The latter could work except that mto would have to be the locative form mtoni.
Mtoni mna mamba. / Mna mamba mtoni. = There are crocodiles in the river. (Literally kind of like "The inside of the river has crocodiles.") This m- here is the class 18 verbal prefix. Class 18 is a locative class indicating internal location and it would require the locative noun mtoni.