Three sheep and four goats have been sprayed purple with an antiseptic substance in a "mindless" attack.
As well as spraying the animals vandals wrote the names "Chris W, Darren and Lez" and obscene graffiti on trees and property at Old Bells Farm in Hordle, Hampshire, on Sunday.
The farm helps children and adults with special educational needs.
Farm owner Ann George said the animals had been left "traumatised" and could be stained for months.
"Why do people do mindless and senseless things to poor animals? It's abuse," she said.
Mrs George added the animals had been sprayed in the face and "intimate areas" which was causing irritation.
The animals are used with people with Aspergers and autism - there are two live-in students and has other children and adults come to the farm for day visits.
"It not only traumatising for the animals but also for the students who live here. They just can't understand why someone would hurt their animals," she said.
Police said they were investigating the incident, which happened some time between 11:30 and 17:00 GMT, as criminal damage.