By the time you notice Umgaruna, it's already too late. When you turn and flee, Umgaruna cranes one of its long necks forward and catches you in an instant. Even if through some miracle you manage to escape one of Umgaruna's heads, another one will catch you from a different angle. If you spot this creature, it's best to take the brave approach and simply prepare for death.
See Frenzied Umgaruna.