President Donald Trump's eldest son appeared to liken migrants to dangerous animals, his latest controversial remarks on desperate people seeking to enter the US."You know why you can enjoy a day at the zoo? Because walls work," Donald Trump Jr wrote in a post on Instagram.Trump's father has doubled down on his pledge to build a wall on the border with Mexico, leading to a partial shutdown of the US government.The president himself has described some migrants in similar terms, saying in May last year that: "You wouldn't believe how bad these people are, these aren't people, these are animals."The Instagram post by Trump Jr - who has previously drawn fire for comparing Syrian refugees to a bowl of potentially deadly Skittles candy - drew criticism on social media."This is racism on steroids," one user wrote on Twitter, while another tweeted: "The only zoo animal is Junior himself."