On Rugrats, Reptar was an obvious parody of Godzilla, but I wonder if maybe it was also in response to the popularity of Barney. It's probably more of a coincidence, as Barney and Friends premiered the same year as Rugrats (even though a few Barney video tapes had been released), but it is rather interesting that the stars of Rugrats were big fans of a dinosaur, while at the same time a (much more friendlier) dinosaur character got his own show and became wildly popular with little children.
And on Rugrats, was Reptar supposed to appeal to little kids (despite all his violence), or was it a case of little kids liking violent things? I've read that there was an episode of All Grown Up where it's revealed that the characters stopped being fans of Reptar, only remembering him from when they were really little. This makes me wonder if they outgrew him (or felt they did), if Reptar actually lost popularity in general, or if for whatever reason (loss of popularity, a controversey of some sort) they just stopped making new Reptar things just before the babies matured.