Lena Dunham made a difficult and painful, but ultimately correct choice to relocate the terrier mix she'd rescued ... at least according to the people who tried to treat the dog's aggressive behavior. 

Lena's been catching heat from BARC, the Brooklyn animal shelter where she adopted Lamby. BARC was pissed to find out she no longer has the pooch ... which she says she had to give away due to behavioral issues. The shelter denies he was aggressive, calling him as "mild-mannered and very well behaved."

To be clear ... Lena didn't abandon Lamby altogether. Instead, she sent him to THE ZEN DOG in L.A., which specializes in working with dogs with behavior issues. The facility's owner, Matt Beisner, says Lamby would get scared and then aggressive every time someone tried touching his neck or head. He says they treated him for more than year, and during that time he bit 3 staff members ... drawing blood each time.

Beisner says he doesn't think BARC saw Lamby's true colors. He says it's common for dogs to act differently inside shelters, and stands by Lena's decision. He adds ... one of his former staffers ended up adopting Lamby, and the dog is doing much better now.

It's clear, Lena was very torn about relocating her dog. She set the record straight after BARC and animal rights groups lashed out and accused her of animal cruelty.

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