^ Lol...I just call it the "M" when referring to it with my wife and brother (who drives an e60 m5)..."my car" when talking to friends (and let them be the judge)...and occasionally "the beast" when writing about it here on the board.
I remember people calling the E34 M5 a beast when I had mine. That said, this was back in '01 and as the E39 M5 was out then I don't know if people had carried over something germaine to the E39 in particular or if this was carried up to the E39 from the previous generation. I tend to think the latter. The S38 while having less HP and torque then the S62 had a real unique "on cam" feel, rough idle, and overall the E34 chassis was a little more course than its successor. I see the term apropo to the E34.
I find myself referring to it as "the 5 series" or "the BMW" to friends or acquaintances i talk to.
On one hand its because i drive a beater Golf Daily and so if i talk about doing something to "the car" they don't know which one i'm talking about, on the other if i refer to the M5 as simply a "beast" or "the M5" i feel a pretentious arrogant twat even though its worth less than a 3 year old Mondeo and there are plenty of other cars out there ready and willing to out perform it!
Tbf, I tend to call mine either my car or the M5, but I guess that's just without thinking about it. I think if I called it the Beast, especially around people, I'd get some funny looks, the kind you get when you have just dropped a proper fart and everyone knows it's you.
The best feeling is when non-M5 and/or non-BMW owners call it "the beast". Had it happen couple times now.
Every time it happens, in my mind I quickly spool back every possible conversation with that person and try to remember if I ever said "the beast" which transferred over to them. But I don't say it around others.... so Goood Feeeling.
Myself I think of "beast" as 800+hp. Though if others call it 'the beast' it might have something to do with the essence of the car.
Personally I just call it The Machine around others.
My sister calls it The Black Panther.