I find that carrots work really nicely. Also for begginers that dont have access to a ghillie suit. Press x and crawl uo to them with said carrot or berry in your 0 slot. As soon as u get close enoigh press E to feed is and then mount it. Every time it bucks press e again to calm it down. Be wary when approaching however because if it gets spooked and bucks you, you will be unconscious for a bit.
To tame one of these creatures, you would need berries or crops. The best is just Rockarrot. Approach one with the feeding item on the last item slot. Feed it and ride it. You'll need to press the action button (E for PC players) for everytime they stopped moving or the icon on the right flashes, prompting you to use it.
To get a 100% taming efficiency on this creature, avoid spamming the buttons and carefully timing your action button. Make sure that your surroundings are clear and there are not too many threats. They often run off into random areas that tends to be near a Giga territory.
They often spawn in large open fields so that is best to look for one. Good luck and have a good one.
Quick tip: I was trying to tame one at my house and it bucked me off and my other tames thought it was hurting me so they killed it. Long story short, put all your other tames on passive if you want to tame it at your house.