MILLSTONE — Dog trainer Scott Krzywicki imitated a Trenton Thunder baseball player as he swung a blue bat inside a training room at Shelly's School for Dogs.

Seconds after the bat hit the green rubbery floor, the Thunder's third generation bat dog, Rookie, was there to snatch it.

"Good boy, Rookie," said dog trainer Shelly Leibowitz as the 1-year-old golden retriever ran with the bat clenched between his teeth. "Drop it, boy."

While pitchers and catchers don't report for another few weeks, the Trenton Thunder bat dog Rookie is already deep into spring training for the 2015 season.

Rookie, who was named last January by fans and supporters, is preparing to carry on his family's bat dog legacy with the team.

He has attended the training school, located at The Green Leaf Pet Resort & Hotel, since May and will join his father "Home Run Derby" at the Thunder's home opener against the Portland Sea Dogs on April 16 at 7 p.m.

"You can see how much he loves that bat. It's his little pacifier now," Leibowitz said, watching as Rookie circled the training room with his tail wagging. "This is the day in the life of Rookie, and he's real happy doing it. He's such a sweet dog."

Rookie visited Thunder games last season to greet fans under the supervision of Eric Lipsman, Trenton Thunder senior vice president of corporate sales, with whom both Rookie and Derby reside.

While in training, Rookie typically stays overnight at Shelly's School for Dogs four days at a time. After two days away from him, Lipsman predicted Thursday that his furry companion would be excited to see him.

As Krzywicki let Rookie off his leash in the training room, Rookie barreled toward him at full speed.

"How's my good boy?" Lipsman asked as Rookie leaped off the floor and then stood on his hind legs to greet him. "I missed you too, Rooks."

Rookie's grandfather "Chase That Golden Thunder" was the first dog to fetch bats exclusively for a minor league baseball team. He spent 11 years with the Thunder before his death in July 2013.

Derby joined the Thunder in 2008 and will accompany his son this season fetching players' bats. Lipsman will bring Derby back to school next month for refresher courses, he said. Rookie is slated to continue in the family business and work in tandem with Derby this season once fully trained.

Leibowitz started with basic obedience skills before refining Rookie’s training to fetch bats at the 56-acre Millstone facility, he said.

"Before we start playing with the bat, I like to have some good basic control, so we build a foundation by teaching him to heal, to sit, to come, to stay, to down, and then we take it a step further," Leibowitz said. "The whole time we’re building drive for him to retrieve, and we try to work him around as many distractions as possible."

Rookie is now ready to put his retrieving skills into context at Arm & Hammer Stadium after eight months of prep work, Leibowitz said.

“When we feel real comfortable all around here, then we take him to the stadium and start working with him,” Liebowitz said. “Even if he does well in the stadium, the big step is when the stands are full with people clapping and screaming."

Lipsman said he is confident Rookie will handle the pressure of the bright lights when the time comes.

“If you look at this picture of Chase and Derby, Rookie looks so much like Derby there,” said Lipsman, pointing to framed photos of the dogs hanging on the walls in the lobby. “He definitely has the genes. They’ve been handed down from Derby to Rookie.”

Nicole Mulvaney may be reached at nmulvaney@njtimes.com. Follow her on Twitter @NicoleMulvaney. Find The Times of Trenton on Facebook.