These are tameable but not breedable. To tame, put down a fish basket by it and capture it. Then go to your base and put the basket on your hotbar then click it. It will be on your floor and friendly. Since they aren’t breedable, they are a bit useless but that’s how you tame them.
When you tame these guys far from home, keep in mind that the Megalosaurus CANNOT pick them up in its jaws when tamed. This caused me a long hike home. Keep it in the basket until you’d like to release it. More testing required to see if any mounts can carry it when tamed!