It could be field maple. I can't see that they would be toxic to horses, - are Sycamore - the same family?

Yes Sycamore is Acer pseudoplatanus.

However, Red Maple/Acer is poisonous to horses

http://www.agnr.umd.edu/news/article.cfm?id=0fbcf86f0a5a5a8f00808e07078e1e43

So you do need to know exactly what you have got Freedom, on and welcome to GoY.

We have a whole line of those trees where |I live by the road, and I am always pulling them up.

It doesn't look like Red Maple to me - red maple has the very distinctive Canadian emblem maple leaf shape and has a silvery blue tinge to the underside of the leaf.

Neither the Blue Cross nor the BHS list Red Maple as a problem in the UK and it is, in any case the wilted leaves in autumn, rather than young growth, that are potentially toxic.

The BHS might be able to help you with an official identification - or try your local botanical garden.

As a sideline - if it was Red Maple you could develop a lucrative business as RM trees sell at about £30-40.

Those leaves will go green as they age.

The adult trees are higher than the 3 storey blocks of flats we have here.