The ponies would probably be terrified at first. The Doomguy, on the other hand, would be confused by these small, colorful creatures. But eventually, they might find common ground. For instance, they could both be fighting for survival in a strange world. The ponies could offer their magic to help the Doomguy, and he could offer his combat skills to protect the ponies.
The interaction between the characters in a MLP FIM and Doom crossover fanfiction would be full of surprises. The ponies' personalities are so different from the Doomguy's. Celestia and Luna, as the rulers of Equestria, might try to bring some order to the chaos of the Doom world. They could use their powers to hold back the demon hordes while the Doomguy goes in for the kill. And the ponies would surely be teaching the Doomguy about friendship and harmony, which would be a completely new concept for him.
Well, it could be really wild. Imagine the colorful ponies from MLP FIM in the dark and gritty world of Doom. Maybe the ponies could use their magic to fight off the demons. For example, Twilight Sparkle could use her powerful spells to blast away the imps. It would be a very strange but interesting combination of the cute and the terrifying.
First, know both universes well. For example, understand the characters, settings, and magic systems in MLP and the other franchise you are crossing over with. Then, create an interesting plot that makes sense for both worlds. Maybe a problem that only the combined forces of both universes can solve.
Scooby would probably be really curious about the ponies. He might sniff around them and try to figure out if they were some kind of strange dog - like creatures. Shaggy would be friendly right away, especially if the ponies offered him some food. The ponies, like Pinkie Pie, would be very excited to meet these new and strange visitors. She might throw a big party for them.
The Keyblade wielders like Sora, Riku, and Kairi would bring a sense of adventure and a different kind of magic to Equestria. The ponies, on the other hand, could teach them about the power of friendship in a more direct way. Donald and Goofy might be a bit startled by the ponies at first but would soon get used to them. Donald's magic could be compared to the magic of some of the unicorn ponies, and they could exchange magical knowledge.
Lucy would likely be a bit scared of the Doom Slayer initially. But as she gets to know him, she could start to see his good side. The Doom Slayer might not be very talkative, but his actions would speak volumes. He might save some of the Fairy Tail members from danger, and they would start to trust him. Happy might try to make the Doom Slayer laugh with his silly antics, though it's not likely to be an easy task.
I really like 'The Avengers in Equestria' which is a part of 'mlp fim crossover fanfiction'. It has the Avengers characters somehow transported to the world of My Little Pony. It's full of action and also shows how the ponies and the superheroes deal with their differences and work together. It's a fun read for fans of both franchises.
The Pokemon could befriend the ponies. For instance, a friendly Bulbasaur might help the ponies in their gardens by using its grass - type abilities. The ponies, in return, could offer the Pokemon shelter and food. They could also learn from each other. Ponies could teach Pokemon about their culture, like the different pony tribes and their traditions, and Pokemon could show ponies their unique battle strategies.
The characters could interact through shared quests. For instance, if there's a dark force emerging in both worlds, they might work together to defeat it. Link could teach the ponies some combat skills like sword - fighting (maybe in a pony - friendly way), and the ponies could teach Link about their unique magic and how it works in Equestria.
The characters could interact through a portal that connects their two worlds. For instance, Applejack could enter the Undertale world and meet Toriel. Applejack's honesty and hard - working nature might impress Toriel, and they could exchange stories about taking care of others (Applejack on the farm and Toriel with the lost children).
The characters could interact through magic exchanges. For example, Hermione could be fascinated by the magic of the ponies, which is so different from the wizarding magic she knows. She might try to study it and figure out how it works. In return, Twilight Sparkle could be interested in the books Hermione reads and learn about wizarding knowledge.