The first purported "feathered dinosaur" known after Archaeopteryx lithographica was Sinosauropteryx prima, described in 1998 (Chen et al., 1998).

Previously, assumptions of affinity to birds had been made, and in some cases backed up by mathematical based parsimony analyses of character-taxon datasets. If it was close to birds, if they have hips like birds, shoulders like them, and it seemed a folding arm mechanism argued John Ostrom in the mid-1960s, they should also have feathers.

But this idea was heavily resisted by stalwart ornithologists who maintained the earlier position that birds were probably descended from some more lizard-like animal.

One reason was the idea that feathers developed from a segmented scale morphology, there were numerous Triassic lizards that seemed arboreal, and it meant all those pesky dinosaurs could stay their normal, cold-blooded selves, giant lizards in the muck.

This is all a bit rhetorical, but is pretty well covered in Pat Shipman's Taking Wing, perhaps the best introductory book out there on the subject. The gist of it is, that early ornithologists resisted, through the 1960s into the early 2000s the very idea that fully developed feathers found on various fossil dinosaurs were of in fact dinosaurs instead of birds; and if the data were ambiguous, instead they were just dinosaurs and not birds.

Though many dinosaurs have been described with simple, down-like basally branching feather-like tufts, like Sinosauropteryx (Currie and Chen, 2001), the sheer seeming unavian-ness of the animal (short, squat forelimbs, forward-directed pubis, LONG tail, etc.) implied that maybe, just maybe, the integument was mistaken, or convergent.

The resistance to the spread of the idea is largely limited to certain ornithologists, creationists, and only a few workers who have more of a beef to methodology than conclusion, unlike the others listed. Several workers, especially Alan Feduccia and later Theagarten Lingham-Soliar would start floating the idea that perhaps these weren't feathers at all, but the remains of a thickened dermis, the remnants of which were a cross-hatch of collagen fibers that just managed to look like feathers.

While evidence continues to rule this out (these structures are clearly distinct and not an irregular cross-hatch, form bundles with distinct central raches, melanosomes discovered on them show color, and that there's no evidence whatsoever of aquatic habits, among a few others) (Smithwick & al 2017).

The early 1970s into the 1990s were the strongest era of resistance, but this all changed with the discovery of a siltstone quarry near Sihetun in Liaoning Province, where apart from Sinosauropteryx a clear progression of diverse feathered clearly-non-avian dinosaurs were found, as well as birds, showing that similar preservation reveals similar integument in diverse groups of near-avian and basal avian dinosaurs.