Where should "imply" be placed in this formalisation?
Here is how to break your statement up:

If (any house is green) then (there exists a horse that is white).

An equivalent statement would be

If some house is green, then some horse is white.

The condition is &quot;there exists a house which is green&quot;, and the consequence is &quot;there exists a horse which is white&quot;.
If we wanted a different quantifier for the condition, we would change it to

If all houses are green, then some horse is white.

As you mention in the comments, this is more of a question in the semantics of the English language. This doesn't necessarily make it a bad question, since it highlights the need for a logical formalism which transcends such linguistic difficulties.
You can formalize the original statement as
$$
(\exists x \, \mathit{house}(x) \land \mathit{green}(x)) \to
(\exists y \, \mathit{horse}(y) \land \mathit{white}(y)).
$$
The prenex form is
$$
\forall x \exists y \, (\mathit{house}(x) \land \mathit{green}(x)) \to (\mathit{horse}(y) \land \mathit{white}(y)),
$$
or equivalently,
$$
\forall x \exists y \, \lnot \mathit{house}(x) \lor \lnot \mathit{green}(x) \lor (\mathit{horse}(y) \land \mathit{white}(y)).
$$