Wild Shape (PHB Pg. 66)
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
Wild shape requires that a druid has seen an animal before being able to shapeshift into it.
Is there anything stopping a druid from combining animals to form a new wild shape?
Yes, the rules stop this simply by how the ability works. The ability just doesn't provide any way to choose two forms. It specifies one ("a beast that you have seen").
Without something giving a druid the ability to meld beast forms, the druid had as much ability to shift into a hybrid form as a random peasant does — none.
[dnd-5e][wild-shape] didn't turn any up. Perhaps a good new question for the site!
– SevenSidedDie
Nov 03 '17 at 02:36
A druid can only assume the shape of a beast. Any combination of two animals would be a monstrosity.
Beasts include “all variety of ordinary animals” (MM p. 6) while “Monstrosities are monsters in the strictest sense—frightening creatures that are not ordinary, not truly natural.” (MM p. 6)
Even more to the point, later on the same page:
Some [monsters] are the results of magical experimentation gone awry (such as owlbears).”
A magical experiment to combine creatures results in a monstrosity not a beast — and thus would be an invalid form for a druid to assume.
Back in Classical Lit they taught me that the Greeks considered things “monsters” that did not fit into proper categories, that is, they were “neither fish nor fowl.”
Many of these classical monsters are represented in the Monster Manual and they, indeed, are all classified as Monstrosity: centaurs, chimeras, cockatrice, griffons, harpies, hippogriffs, manticores, medusas, and sphinxes.
Natural hybrids (such as mules or hinnies) are a different case entirely. A druid would simply need to have seen such a hybrid to assume the form.
By RAW: No you can't wildshape to a hybrid beast
The wording of the ability states
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
emphasis mine
This implies both that the the form is of a single beast, and that the beast has to be one the character has seen not one that they can envision by morphing 2 beasts together in their mind.
This does though leave a possibility of being able to transform into a hybrid beast that you have already seen, but it would have to be of type beast
Owlbear is a hybrid beast, for example. However it is typed monstrosity therefor not a beast of itself and cannot be selected as the form to take.
The Mule on the other hand, also a hybrid beast, is typed as a beast. Therefor the mule can be selected as the form to take.
Yes there is something stopping this, the rules:
Wild Shape (PHB Pg. 66)
Starting at 2nd level, you can use your action to magically assume the shape of a beast that you have seen before.
You have to have seen it, so it cannot be something you have made up.
