I don't think that the mules are feared, rather they are strong animals that are difficult to handle, and can get out of control. The Terumas Hadeshen (Pesakim 105) points out that it is obviously possible to raise them without incident, as Rebbi would otherwise not have had them in his house due to the Halachah that one should not harbor a damaging animal in his domain. However, R' Pinchas Ben Ya'ir was stringent in this matter, and therefore held that these qualified as animals that one should not have in his house due to their damaging potential. However, these mules are not in the class of true beasts such as a lion, tiger, etc..

All the best,

Yaakov Montrose