NBL really dropped the ball not taking up former trademarks.
They actually do still have one of the old trademarks, it's just a particular depiction of the word "Devils". Not sure, but I assume it is/was for use on some retro stuff, as they also hold a few other old ones.
Ultimately, I think they'd still have some freedom in choosing a mascot.
The 6's have a Magpie? Breakers have Cheeky the Kea, etc.
If the name was otherwise not specific to a particular animal, you could still have "Tassie the Devil" as your mascot. Provided you're otherwise careful with your branding, you would probably get away with that. (Although mascot-based merchandise would need to be especially careful.)
You've also got to remember that new teams tend to strive for "new" names, rather than defaulting to obvious choices. Tasmanian is fortunate in having two unique fierce creatures (although hunting one to extinction probably not a smart move.)
In WA we have the Black Swan, and whilst it's beautiful and our widely used state symbol, none of our teams carry the name.