Orgyia antiqua

I found this caterpillar whilst gardening today. I had never seen such a strange one before.

It will eventually become a moth called Rusty Tussock Moth.

They are easily recognized by their horn-like tufts of hair-like setae. There are 4 toothbrush-like tufts along the back, and hair pencils that project from the sides at the front and at the back. The body is dark grey to black, and there are red tubercules along the sides and back. They have defensive glands at the back, and wipe their setae against them to charge them with toxins. They grow to about 30–40 mm, females being considerably larger than males. In the UK, caterpillars can be found between May and early September