Bump on back legh

Hello I noticed this bump on the back leg of my Aussie shepherd. Anyone know what this is?

Hello!  This is an interesting picture - I am looking at it on a phone, which may not show the whole thing, but from here - if you'd been deliberately trying, you couldn't have framed it better to disguise the position on the leg or the size or shape of the lump.  This is relevant because the lump looks a little bit like a normal carpal pad from a certain angle, although these are found on forelimbs and not hindlimbs.  It could also be a relatively flat bald area, for example as caused by ringworm or demodex, or it could be a domed, hairless area, such as a callus on a weight bearing surface.  Possibilities not yet mentioned include cancerous or non cancerous lumps such as lipomas, or inflammatory lumps such as infected bites or foreign bodies.  So;  what to do?  As with all lumps (search 'lumps' in the blog for more information), it may not be diagnosed without a biopsy and certainly does need to be shown to the vet, in order that they can come to an agreement with you as to when, how and whether this could be acheived.