It's mostly delighted or pleased, although it can occasionally mean the opposite, especially in the military. I believe it started off as "proud." This bunny is delighted and proud of his new rug.
I'm amazed he knows how to use a litterbox and not just poop everywhere. That's probably how he earned his nice rug!
Chuffed is slang for very pleased in the UK. More specifically it's when something good has just happened to you that's causing you to grin uncontrollably like a lunatic.
As my crude father would say: "I'm chuffed to buggery!"
Although buggery is a bad thing on its own, when combined with chuffed it's like a +1 enchantment instead.
If it were contained in some way, that would make sense to me, but it's free to roam as far as I can see. As for the hay mat, that still seems strange to me, but fair enough.
Rabbits I’m not sure. Cats naturally want to bury their leavings, so they actually train themselves just fine. Show them where it is, keep it clean, and 99% of the time that’s that.
You just toss the cat into the litter box after setting it up and they're like "this is fucking rad, dude! Thanks! Now leave me alone to do my bidness".