i do think the marauders were a good group of friends (yes prongsfoot were definitely THE duo but that happens in groups and it doesn’t make them bad friends). however, if we have to say one of them was “excluded” it would be remus lupin and not peter pettigrew. i see a lot of posts on how peter was left out and that’s the reason for his betrayal but i don’t really believe that.
the marauders became illegal animagi for remus but it wasn’t something he was a part of. james, sirius, and peter shared that between the three of them—the entire process of becoming one, being in full control during their full moon adventures and all that. after the night of the full moon, again, the other three spent time together and remus was more alone.
we also see in snape’s worst memory that peter went with james and sirius but remus was the one who stayed back, so i imagine he usually avoided directly being a part of those activities while peter didn’t. when harry overheard mcgonagall and the others at hogsmeade, they mentioned sirius, james, and peter. no one even said remus’ name (yes i realise that could just be a plot point but even in lily’s letter to sirius, she mentioned peter and nothing about remus).
another major point is that remus was the only marauder kept out of the loop on the whole secret kepeer thing (also i HATE that entire plot line with a passion because none of it makes sense anyways). he was the only one who had no idea of the secret keeper switch and when james died, all three of them (james, peter, sirius) were aware that peter had been the spy. remus, on the other hand, spent thirteen years not knowing any of that.
remus spent his entire childhood fearing that he’ll never make any friends and he will forever be shunned but he did end up finding these three idiots who not only accepted him but went so far in his aid. he naturally had major self esteem issues and when someone is insecure like that, they usually just distance themselves and stay away for so many different reasons (fear of scaring their friends away, being too much, people judging them, being seen as they see themselves, and so on and so on). it’s not his fault and it wasn’t theirs but when it comes down to it, i genuinely believe remus was the “left out” marauder (if there has to be one, that is … note that i’m not arguing that they didn’t love remus, just that he was the one who was more separate (?) from the others).
as for peter, it makes sense why he would feel that way, too. it could be his own insecurities to an extent but also when remus was absent and it was just peter with james and sirius, it would be very easy to feel left out. the whole “there’s a duo in the trio” thing but then duo being james and sirius was impossible to compete with for anyone, not just peter.