#mothim

Mothim and Leavanny - Leavanny I'd an incredibly loyal pokemon and they mate for life. Mothim aren't. They go around looking for wormadam or any willing bug to mate, and they move on to the next. What's going to happen if you breed the two is that you're going to have a really heartbroken and depressed leavanny.

ANY SMALL POKEMON WITH ANY LARGE POKEMON

Any monogamous pokemon with a non-monogomous pokemon

Male Wailord and female Skitty - I can't believe I have to say this. But, laying the egg can be very painful due to the size difference, mating can kill the skitty, and the skittens (baby skitty) that come out of it are prone to health problems like obesity and their legs bring too small to handle their larger height/size. Still, because I'm a biologist first and it does happen, I did make a post going over the breed. Honestly, don't breed wailord with most pokemon. Just leave the wailord to breed with each other

Eelektross line and the Jellicent line - the Eelektross line might randomly release electricity during mating, which can really hurt Frillish/Jellicent.

Zangoos and Seviper - Why would you even try that?

Magikarp and Gyrados - same species, ik, but the height difference is a problem. They might not even register each other as the same species, tbh. Same with Feebas and Milotic