Like many American kids I grew up Warner Brothers cartoons. I imagine I was also not the only one to be impressed with how fluidly the creative personnel behind the shorts handled the interaction of a large stable of cartoon "performers," having many of them cross one another's paths in stimulating ways-- at least in the Golden Age of the 1940s. But what were the first Warners crossovers, starting with their genesis in the early 1930s?
Warners' earliest cartoon-star was Bosko, who combined elements of the popular 1920s character Felix the Cat with aspects of a minstrel-show Negro, but he had no crossovers with other characters. One early cartoon, "Bosko's Fox Hunt," had the hero pursuing a tricky fox, and for a moment I thought there might be a link to Warners' second attempt at an animated lead, "Foxy," who was basically Mickey Mouse with fox-ears and a fox-tail. But the fox in "Hunt" doesn't walk on two legs like Foxy, and at most the former might have been an embryonic version of the later (and unsuccessful) vulpine protagonist.
1933 saw Warners unleash numerous parodies of Hollywood stars, which they would continue to release over the years. But none of these count as crossovers, since they deal with actors, not characters as such. However, the same year saw the debut of THREE'S A CROWD, Warners' first "book characters come to life" short. Alice from "In Wonderland" is more or less the star, as she runs around the bookstore interacting with other famous figures-- the Three Musketeers, Uncle Tom, and Robinson Crusoe with his Man Friday (who's rendered as another Negro, not as the Carib Indian of the book). Mister Hyde provides conflict by kidnapping Alice, but he's foiled by Tarzan, Robin Hood and the Musketeers.
The first crossover of featured characters would seem to be 1935's I HAVEN'T GOT A HAT, but only if one regards one of the characters as a "star-in-the-making." HAT is the debut for short-lived starring character Beans, a mischievous black cat (again with a little Felix design-mojo) and also for Porky Pig, not yet in his final design but already sporting the trademark stutter. A couple of later cartoons team up Beans and Porky as equal partners (could the writers been thinking of "pork and beans?"). However, in HAT Beans gets the lion's share of plot-attention, even though Porky steals the show with his stuttering reading of "The Midnight Ride of Paul Revere." After a few years it was clear that Porky was getting the plaudits of cartoon-loving audiences, and Beans was phased out-- though in the 1940s Porky would gain considerable fame for his team-ups with Daffy Duck and a few other luminaries.
1937 debuts Daffy Duck, though not yet under that name, as the adversary for Porky in PORKY'S DUCK HUNT. Though over the years Daffy went through many metamorphoses, his persona in "Hunt"-- that of the "crazy, darn-fool duck"-- is definitely part of his core appeal, rather than a fleeting experiment. So this was an unquestionable crossover between an established star and another star Warners hoped to launch as a franchise-figure. A year later Daffy would be the star in DAFFY DUCK AND EGGHEAD, given that Egghead-- sometimes seen as a transitional figure for Elmer Fudd-- never really caught on with audiences as a lead character.
In the same year, Porky also appeared in PORKY'S HARE HUNT. Beck and Friedwald think that this short might have started out as another duck hunt, albeit with the substitution of a crazy rabbit. I would consider this daffy bunny to be only a stepping stone to Bugs Bunny, but not close enough to be deemed a crossover participant. Both Bugs and Elmer Fudd would take their definitive shapes in 1940's A WILD HARE, and this team-up would prefigure the way many other animated regulars would start interacting throughout the forties.
Unless I missed something in my survey of Warners cartoons in the thirties, I found no instances of crossovers with either fairy-tale or nursery-rhyme characters. 1940 brings A GANDER AT MOTHER GOOSE and 1942 gives us FONEY FABLES, but both of these cartoons are just a bunch of blackout gags, in which the folklore-characters don't interact with one another. I feel sure there had been such fairy-tale crossovers at other companies, but Warners didn't do any in its first decade.