There's formaldehyde in a lot of make up and over the counter products as well like cleaning products and facial care products for people.
But why would you want to inject in into your horses hooves?
The stuff you buy at the feed stores works very well...especially that thick green one that for the life of you you can't get off your skin (can't remember the name) I've also used a product made by Melaleuca called Solumel that works wonders and is not poisonous at all.
That is just spooky. It is skin permiable, and not something you want go purposely introduce into anyone's system by any means. It is known to harden nail or hoof material if painted on the surface - but is not used for that purpose. What nut case would inject anything into a hoof for the purpose of eliminating thrush?