A changeling’s mimicry and magic can allow it to take on all manner of disguises, including nonponies, though these are more difficult to assume.  The larger a disguise the more difficult it is to maintain, and a changeling’s disguise can never exceed its own mass so while a changeling could become a giraffe it would be a rather light one.

A giraffe can become a changeling following the same process as most other creatures.  The results tend to be lanky and awkward however.