Erebus is designed around the concept of the Ageing Badass. He's the grumpy old veteran with a million reasons to hate everything around him, and only needing one reason to snap at the nearest thing in reach. I tried to embody that in these images.
With the younger one, he's got a shorter temper and more violent attitude, but is also impatient [hence him snapping just at the sight of Crusher, but breaking off when Crusher disappears into the undergrowth and the Raptors come into view, switching his focus on them instead]. As he gets older, his fuse gets longer, but his mood deteriorates until he is eventually locked in a constantly bad mood. players of the old ISS will probably remember the incident where he launched a log across a river, nearly crushing a bunch of Dromeosaurids, just because of some minor irritation.
And it only gets worse

As for choosing the Trike, reason is simple: Out of the major herbivores, none is more threatening in the average Dino lover's mind than an angry Triceratops. An armoured Rhino the size of a bus. Not as sluggish as the Anky, but just as capable of immense destruction.