3) To my fellow science nerds: one of the hashtags for this gif image is #Birds, how does that make you feel? 
#Birds?
Perhaps the best description of how I feel about that is the following emoji:
The word "PTEROSAURS" exists FOR A REASON, people...
Sigh...okay, moving on.
1) It has be postulated by many authors, including myself, that Sierra might be some kind of psychopath or sociopath (a person without emotional empathy for others) based upon his words and actions during the seventh film. Including his willingness to kidnap Ducky and then to discard her to what he assumes is her doom. What do you think?
2) What is your personal head-canon about how these two met up? It does seem that Rinkus knows how to recognize and handle Sierra's more intense impulses to lash out to a much better degree than Pterano.
Despite a childhood of watching LBT VII to the point where I knew it by heart, I never really thought about Rinkus and Sierra's backstories at all. (That fanfic I wrote about Sierra for a prompt a couple months ago doesn't count, as it was hardly a serious fanfic.)
The two flyers do seem to have a close dynamic, though, which is interesting. Although, at the same time, I wouldn't necessarily consider them friends, since there sure weren't any signs of either one being willing to let the other in on the stone's "powers" at all (see Rhombus's GIF).
As for Sierra maybe being a psychopath, I think it's certainly possible based on his actions/words (for example, wanting to feed the Gang to sharpteeth). He's by far the most menacing non-carnivorous villain LBT has to offer, which is something I appreciate when I look back on LBT VII. Non-sharptooth villains were always good, but when they seemed like a real threat, they were even better.
As the real brains of the whole "stone of cold fire" operation, Rinkus was a competent villain, too, bringing a sense of sneakiness that counteracted Sierra's hotheaded tendencies. However, he does bring me to another complaint I must make in the interest of science...
While Cearadactylus and Pteranodon were both on the larger side for pterosaurs, Rhamphorhynchus wasn't even half their size. Except, apparently, in LBT.
I know these movies are far from scientifically accurate as it is, but even as a kid I knew a Rhamphorhynchus shouldn't have been as big as the other pterosaurs. Why did he need to be so big, anyway? It's not like he was the one who ended up carrying Ducky.
But I'll stop my rant here, moving along...
Aside from these two being among LBT's best villains, they also worked for being played for laughs now and then. Sierra griping about Ducky was amusing: "Nope, nope, nope, yep, yep, yep...it's driving me NUTS, NUTS, NUTS!"
(That "nuts, nuts, nuts" part is one of those LBT lines that caught on among my family years ago, and it even still comes up in conversation sometimes.) The way Rinkus cuts Sierra off when he tries to complain in the middle of "Very Important Creature" ("What did you say?" "He said LEAD THE WAY!") made me chuckle, too.
