"A chair is a chair, is a chair, is a chair … but a seat does not necessarily have to be a chair. It can be anything as long it is ergonomically correct. A seat could even be a small and soft Pony on which you can ‘ride’ or sit sideways."
So said the designer Eero Aarnio and low and behold he designed the Pony Chair in 1970. Whilst this chair obviously will appeal to children with its oversized cartoon-like form, it is actually large enough for adult use, if you so desired.
Vertigo Interiors are selling the Pony Chair for under £200 so it might not be an authorised version (USA based sites like Nova68 are selling them for over $2000), but it looks pretty much the same and if you are buying for a child, they probably won’t mind.