Batman and Robin first came across the T-Rex after battling a lesser criminal. Robin thought it would make a good memento, but Batman stated that transporting it alone would attract unwanted attention. Nevertheless, he later relented to place it in the cave. As a visual nod to Batman battling an animatronic dinosaur, the episode "Mean Seasons ... More @Wikipedia
Hover over any link to get a description of the article. Please note that search keywords are sometimes hidden within the full article and don't appear in the description or title.