Swollen hind

[ace123]

Pony has a swollen bum. (literally) This morning there was no pain but felt like fluid from top of his tail over his bum and going down hind leg. Vet said kick or bite and see how it goes as no pain. This evening its not as swollen but some of its gone lumpy ( top of leg ). Kicked out when i touched it, still not lame but snatching leg when walking. Rang vet again, they said the same as this morning and to keep him out tonight then see how he is in the morning.
Any ideas please...
Someone said a possible snake bite .....thoughts please
Thanks sharon