What will happen to an animal if all the mitochondria disappeared ?

The animal would, quite literally, have no more energy and would not be able to function.

Hello! Mitochondria is the power house of a Cell, it generates the ATP needed for Work ie the Energy. It is where Thermogenesis happens and also it is the site where coupling and uncoupling of electrons happens. the breaking down of glucose and converting them into energy is done by the Mitochondria . Now answer your own query , No energy means Death of a Cell ie Necrosis.(apoptosis doesn't happen in this case). I hope this helps :)