.... *Pictures Rowlf getting ticked off at Cookie Monster's antics, suffering Grover's 'help', putting up with Herry breaking things, and Elmo's constant yelling at him.* I just see Rowlf taking it all in stride. Rarely did I see Rowlf loose his cool, like in the Muppet Show where he said a poem and everyone else started joining in. How ever, Kermit on the other hand, is always on breaking edge. He tries to keep things cool, but will join in and even make sarcastic remarks that make him seem so mean, but still loveable.
And besides, Rowlf can't do the facial reactions for when little girls claim Cookie Monster is a letter of the alphabet.
I think it also had something to do with the monsters being in the same color range as him at the time. The TV's back then just where experianceing color if I remember correctly, and Rowlf's brown fur may have classified him as a monster like FuzzFace, Scudge, BDM, FluteSnatcher, ProtoCookie, the Snerfs, and heck, even that green meatball thing that appeared in the C sketch.
I was shocked to know Rowlf made a cameo, but thats nothing. If I remember correctly, a FRACKLE appeared on the Show when bippa boppa or what ever, erm...The shows Mahna Mahna spin off is the best way I can describe him, sang All of us, None of us. The monsters included Cookie, Frazzle, Herry, a few whatnot monsters, and a Frackle. It just seemed weird.