Unless they are the same type they will not have an egg.
Actually, they can. I had a pikachu and a skuntank have an egg, when the egg hatched, it was pichu who knew toxic. Weird, but any Pokemon can have an egg, if it's a boy and a girl. (Just make sure it's not a Pokemon with no gender, only dittos can have eggs with any Pokemon and it has no gender.) By: Blackcat 90 78
Only bring Pokemon of the same gender into the daycare. You can also refuse to take the Pokemon egg.
A girl torterra and a boy same Pokemon/type.
In Pokemon Diamond and Pearl with two compatible Pokemon in the Pokemon Daycare the game will produce an egg every 256 steps. Pokemon left in daycare must be a male and female unless player is using a ditto which is gender-less.
Go to Solaceon Town and there is a daycare on the right. Leave your desired Pokemon and a Pokemon of the same egg group but different gender. It will help to get the Daycare App so you can see when your Pokemon get an egg. Get the egg from the old man outside the daycare. The egg will hatch after a certain amount of steps.
Figure out why they won't have an egg. Some Pokemon can't have eggs. If they're the same species, then they need to be of opposite genders. Some Pokemon like the legendaries are genderless, and hence cannot create eggs. The same goes for two Dittos. Baby Pokemon, like Pichu, Magby, Elekid, etc., can't lay eggs either. Just be sure the Pokemon are of the same egg group and opposite genders (or just check that they aren't genderless/baby Pokemon if you're using a Ditto as the alternative mate).
You get Pokemon eggs by sending two of the same Pokemon or a Pokemon and a ditto to the daycare lady and while they level up they also mate
If you mean BREED Pokemon in Pokemon white, you put a girl and boy of the same Pokemon in daycare, and you will later get an egg from the daycare people.
First, you get a female Pokemon and then you get a male Pokemon (same kind) and then you put it in the Pokemon daycare and then you wait a while
any female Pokemon can lay eggs butonly if you put to of the same Pokemon in the daycare eg. piplup + piplup or if you do piplup and ditto
You can put the same Pokemon with a different gender or you can use a ditto with a different Pokemon that has a gender
Only bring Pokemon of the same gender into the daycare. You can also refuse to take the Pokemon egg.
You get Pokemon eggs by breeding them. They come form the daycare when you put a male and female Pokemon in that are compatible. By compatible I mean they have to be the same egg group.
A girl torterra and a boy same Pokemon/type.
put 2 of the same Pokemon in the daycare 1 boy 1 girl and wait for a egg or put the Pokemon you want in the daycare and a ditto and wait for a egg
Put a Pokemon and a ditto in the daycare at the same time and wait. No legendaries though except for phiones and manaphys.
all Pokemon except legendarys. put 2 Pokemon same species opposite gender or a ditto and any Pokemon in the daycare
Leave two Pokemon of different gender and the same species at the daycare center .To leave two Pokemon at the same time you must have a bike. hope this helps! >_< :) O.o