I recall this very early in classic but I couldn't say for sure. Here is a thread from 2004 with people talking about how they wish this was possible (suggesting that it wasn't before the 2004 patch that allowed all pets to see invis).
---liz-
07-12-2004, 01:00 AM
"I wish that with the zoning pets they would allow us to cast see invis on pets and have them not disapear, as now you can get them buffed and take them with you, having to invis to run through say FV or something just really makes it sad when someone has buffed your pet (despite you not asking) and then you have to kill it with invis."
http://webcache.googleusercontent.co...p/t-62483.html