"Apparently, headman, I haven't been able to get past you by saying, 'Enough, headman, put that aside. Don't ask me that.' So I will simply answer you. When a warrior strives & exerts himself in battle, his mind is already seized, debased, & misdirected by the thought: 'May these beings be struck down or slaughtered or annihilated or destroyed. May they not exist.' If others then strike him down & slay him while he is thus striving & exerting himself in battle, then with the breakup of the body, after death, he is reborn in the hell called the realm of those slain in battle. But if he holds such a view as this: 'When a warrior strives & exerts himself in battle, if others then strike him down & slay him while he is striving & exerting himself in battle, then with the breakup of the body, after death, he is reborn in the company of devas slain in battle,' that is his wrong view. Now, there are two destinations for a person with wrong view,
I tell you: either hell or the animal womb."Yodhajiva Sutta: To Yodhajiva (The Warrior)
http://www.accesstoinsight.org/tipitaka ... .than.htmlThe Upanishads are not considered canonical in Buddhism.
But Buddha spoke of the lower realms which include the animal realm, hell and other such undesirable locations.