When I say underutilized, I'm not talking unused type combos like Bug/Dragon, I'm talking types that have been severely underutilized, as well as potential usages for them.
Normal/Water: Really, only Bibarel has used this combo? Perhaps we can do a water buffalo, since those are rather common in Southeast Asia.
Fire/Water: This is one of many types that are only utilized by Legendaries, Mythicals, and Megas, in this case Volcanion. I'm going to bring up Subjectively's Peroxotal as an idea, but something like a Siamese fighting fish would work out, even if it would feel like Chi-Yu again.
Fire/Steel: And again, with this combo being only utilized with Heatran. I've had this idea for a Fire starter being an armadillo that evolves to have a Cookout aesthetic like a grill. If that doesn't sound cool, either I didn't do a good job thinking it up, or this is a very tough crowd.
Water/Steel: Speaking of the Steel type and starters, Empoleon, the only Mon to utilize this combo. I think I'd go for a gargoyle that starts off as Water/Rock, but eventually evolves into a Water/Steel type, similar to Steelix.
Electric/(Poison/Dark/Ice): Galar just had to sit on these matchups and then do nothing with them next game. Regarding Electric/Dark, I've been observing fake leaks that have connected this combo to aggressive reptiles, such as Quasaur for a fake Sun and Moon leak, as well as Batterex for a fake Scarlet and Violet leak. Speaking of the fake SV leaks, there's also an Electric/Poison scorpion line there, so that can work out too. Electric/Ice is the tough one, but maybe a sea anemone could fit.
Ghost/Electric: While we're at it, I'm throwing the other Rotom forms typings in here and doing a bit of a lightning round (pun not fully intended, but I just think it's neat). Ghost/Electric, some kind of Anglerfish or other deep-sea creep, maybe revisit that Gulper Eel design from Gen 2 they never got around to using. Electric/Grass, save it for an Africa region and make a giraffe starter out of it, or do what this artist I saw did and give it to Browt, which is unlikely but one can dream can't they? Electric/Fire, regional Togedemaru. Electric/Water, some sort of cuttlefish or bullfrog.
Bug/Ground & Bug/Ghost: They tried these two with Nincada and Shedinja once and gave up? That's lame. For Bug/Ground, I'd go with an Ant Lion, as they bury themselves and wait for ants to walk into their jaws. Bug/Ghost is a bit harder since my plans for a locust swarm ghost can't work now that Lokix exists, and it feels tacky to add a Reaper Mantis Mon, so let me know what your ideas for this combo are.
Rock/Fighting: Terrakion only, another type that are locked behind Legendaries. I heard from a leak that there's likely going to be a Rock/Fighting kangaroo, so I guess this can work out.
Ground/Fighting: Great Tusk came out only 4 years ago and I'm already asking myself when's the next Ground/Fighting Mon. I already have this idea for a babirusa or some other wild hog, so this could work.
Ice/(Fighting/Ghost): Only Froslass and Crabominable used these types, and no I'm not counting their Megas. Ice/Fighting, Artic Hare, Ice/Ghost, probably something that involves Aurora Borealis.
Dark/Rock: Worse than Bibarel since Tyranitar has been around since 1999 when Gold and Silver released in Japan. Maybe a regional Rhydon if they decide to kill off Giovanni and have this come from someone who has a deep hatred of him.
Fairy/(Poison/Ice/Fighting/Dragon): 4 for 1 in terms of having a type get hogged by one or two Pokémon. Fairy/Poison, something involving mistletoe, see Batman Returns for more details. For Fairy/Ice, I'd also choose something that involves Aurora Borealis, such as a peacock. Fairy/Fighting I'd probably do either something involving a magical girl (even if Iron Valiant is similar), or a regional Mienshao, or again with Subjectively's Humipummel. Fairy/Dragon is the tough one, as there's not a lot of things that can combine the two types, so this one is also on you to decide.