Do you snore, know someone who does or have access to sample libraries of snoring? If you pitch a snore down, it can sound very animalistic...so I've heard anyway :) They sound more like growls than roars, but could easily be used as layers in a roar.
I recorded mysel...I mean my good friend gave me some recordings just like that, 96 24, which I think work rather well. PM me if they'd be useful to you.
As for other tips, I saw a really good Youtube video where someone used a vocoder to articulate a monster sound.
Reformer by Krotos is free and comes with a free Black Leopard library, which shapes itself to whatever audio you feed it. I don't know quite how it works and apparently it uses AI, but it seems like something between a noise gate that triggers samples and envelope following to me from the demo videos I've seen.
Waves RBass is good for beefing up the bass in a sound to give more threat. Be careful not to over-use though. I have a few sound libraries where the monster sounds employ this technique and the massive amount of bass doesn't work with what sits above it.
Morph by Zynaptiq is great for this kind of stuff as is their other plugin Wormhole. There is Dehumanizer ii by Krotos as well, though that's quite expensive and their cut down version, I forget the exact name, Simple Monsters I think it is.
Other than that, you could layer the same sound with pitch-shifted copies of itself, though you have to get the intervals and number of copies just right to avoid an obvious harmonizer effect and to make sure it's thick enough.
Other than that, layer different sounds with aggressive filtering. I should say that I've never had personal experience of success with this technique, but I've read a lot, both on here and elsewhere, that it's how the monster-making pros do it. It's kind of like a witch's brew.
Take the bass of a waterfall or jet, the mids of the howl of a pitched down baby cry, the highs from the snif of a dog, the pant of a horse, process to taste, let simmer by the light of a full moon and your creation is born.
I read a tip on here that suggested that, in order to make these layers into a homoginous sound, one way is to apply the same pitch envelope to all layers.
Hope this helps.