The "invisible thread issue" in the levelup5 forum continue to plague me, but for whatever weird reason, the thread preview under the forum header (aka that shows the most recent active thread) still shows me the thread, and so I found out you have posted lots of monsters without my knowledge!!!
Otyugh - Really cool, my god that disease is nasty as it makes you poisoned all day. Now that's a disease worthy of remembrance! My only issue, the Swallow bonus action is a little confusing to me. It mentions a "bite" but I'm unclear if I should actually be resolving the bite attack as a part of this action, or if that is just flavor text and I should only do exactly what the entry tells me to do.
Purple Worm - Purple Worm honestly seems underwhelming for its CR 15. The fact that it has to spread its damage out is a major weakness to its offense.... I'm also very surprised that the combat section has this creature attack and "stick around", I felt like this was the perfect candidate for a creature that swallows someone and just leaves.
Rakshasa - Another really fun curse, that is something that will have the players pay attention, and the read thoughts is great!. I will disagree with the combat section again on this guy.... frankly his damage is garbage for a CR 13, the curse is a fun way to mess with people but its not an effective combat weapons against parties capable of threatening you..... Dominate Person is the only effect here that could give you enough "CR 13 offense" to be worth using. Ultimately the Rakshasa has plenty of defense but little offense otherwise.
Tarrasque - Now we are talking! You fixed the biggest issues with the 5e tarrasque, which is the lack of regeneration (seriously a Mr T that cannot regenerate is no T to me!) and the inability to deal with flying creatures. You have solved both in great fashion, I love the earthbinding callback to the 4e Tarrasque. That jump is super cool, this is a tarrasque that gets it done! If I were to change anything (which is more of a preference) is I would drop legendary resistance and just make elite recovery on all the time.... that gives me more the vision of a monster that just never stops, never runs out of juice, just keeps going and going. But again that's more preference than a real issue. The only other one for me, it would be neat if a saving throw success against the Roar did not make you immune to the fear. Aka this is the one creature that is so badass, even if you steady your nerves for a second, its so primal that a second blast will bring the fear tumbling back. I think for most creatures the immunity makes sense, but Mr T could be the exception.
Quick Tangent: A longstanding rules gap in 5e that Dms have to decide on, can you make multiple attacks against something while you are jumping? Do you all decide to clarify that in your book or do you leave it open ended?