Any idea why I can't tame this Beast? Been spamming Tame Beast on it and even dropped my existing Beast. Maybe because its a scripted Beast in the Hunting Grounds?

Because it’s not a beast it’s a tree

Sadly, despite no statement otherwise, you can't tame most "scripted" rares. This guy, the rabid dog in Ogham Farmlands, the larva queen in Infested Barrens, etc. are all untamable despite being categorized as beasts. The guaranteed Boar and Antlion rares that are at the camp in Infested Barrens, however are tameable... so go fucking figure.

If it's a quest rare, you can't capture those,even if it's a beast.

Some "unique" rares can't be tamed unfortunatley. If they're part of a set piece I think.

Get a prowling chimera.