OLD YELLER. BtVS Intertext/Allusion.
In "Doppelgängland," Willow, distressed by being thought too predictable, talks with Xander and Buffy.
WILLOW: (re banana) I'm eating this now. It's not lunchtime, I don't even care.
XANDER: Willow, did you remember to tape Biography last Friday?
WILLOW: (absently) Uh-huh.
BUFFY: See, I told you. Old Reliable.
WILLOW: (sourly) Oh, thanks.
BUFFY: (taken aback) What?
WILLOW: 'Old Reliable'? Yeah, great. *There's* a sexy nickname.
BUFFY: Well, I-I didn't mean it as...
WILLOW: No, it's fine. I'm 'Old Reliable'.
XANDER: She just means, you know, the geyser. You're like a geyser of fun that goes off at regular intervals.
WILLOW: (disgustedly) That's Old Faithful.
XANDER: Isn't that the dog that, that the guy had to shoot...
WILLOW: (incensed) That's Old Yeller.
Old Yeller was a 1957 western film. The IMDB offers the following brief account:
Young Travis Coates is left to take care of the family ranch with his mother and younger brother while his father goes off on a cattle drive in the 1860's. When a yellow mongrel comes for an uninvited stay with the family, Travis reluctantly adopts the dog. After a series of scrapes involving raccoons, snakes, bears and all manner of animals, Travis grows to love and respect Old Yeller, who comes to have a profound effect on the boy's life.
Xander is recalling the film's tear-jerking ending, in which Travis must kill the dog. Go here to read an article arguing that Old Yeller is "one of the most disturbing 'children's movies' in cinema history."