So I was moved after watching a Deadly 60 about Rhinos and poaching. So I came up with the idea of an Anti-Poacher, who protects animals. A beast master, not sure of beast, favoured enemy is humanoids. How does it sound?

Should be a worshipper of Mielikki if you're in FR.

Yeah. Or Malar

Any ideas for beast? Is Savannah more grassland or forest? 

I'd have the beast be a typical trophy animal … a bear, a fox, an elk, a hawk, an eagle, …  Of course an owl is probably the go-to beast.  In a savannah terrain you might want to have a rino, an elephant, an eagle, a large cat, ...

It may be kinda dark, but would you want your anti-poacher ranger to "hunt" the hunters?  Use traps, have a legion of dogs to "run" his game and flush them, keep trophies of the 'game' he has caught (which I would think would be the tunic and weapons of each disarmed offender)?

Maybe not keep the trophies, by yes, that sounds cool. Do think that I should try and homebrew a baby rhino as if I found then saved it, or just get a panther or wolf then re-skin it as something else?

A panther would be the go-to pick for must folks.  A rhino gives you interesting flavor and problems to solve.  You can ride a rhino or give him some mongo saddlebags.  Keeping him fed, on the other hand, will be not just a job, but an adventure.  I think given the choice, I'd go with the panther.

All a rhino needs is grass and small shrubs. So what then?

I would fear he would wander off seeking better grass or a river to drink from and cool off.  To keep him you'd have to tie him up or feed him well.  And with any animal, remember they have to be exercised from time to time.  I often have to avoid riding my horse to the final destination because I can't leave him tied up for a few days while I'm dungeon crawling.  I have to leave my horse in the nearest settlement and pay for his upkeep.

Ok, that makes sense. Thanks for this, it is really helpful.