I have 3 unlimited hoards, 1 complete hoard. The unlimited ones are weather ponies, lightbulb spiders, and the "Tron" horses (or whatever they're commonly called). The complete one is 5 of each of the elemental dogs from 2011.
Why do you hoard them?
I hoard the weather ponies because I used to be a meteorology major, they used to be crazy-easy to get, and I loved the multi-stage change in the 2010 ones.
I hoard the lightbulb spiders because I thought it would be funny to have a group full of them.
I hoard the "Tron" horses because I like the techno-design and because I thought they were music notes when they were foals

I hoarded the elemental dogs because I liked the patterns
How large is your hoard?
Weather ponies: 481, Spiders: 72, Horses: 190, Dogs: 20
What is your goal for the hoard?
No goal for spiders or the horse litter, but with the change of the Equimagine lines I'm going to stop at 500 for the weather ponies (unless some new weather or snow ponies come out in the near future). The dog litter is complete at 5 of each outcome.
Do you overpay for hoard pets/items?
No. I'm kind of stingy that way. I keep them on my WL so I can get them from the pound or so there's something easier for people to find for CS lights on/off on October. When I started my pony hoard, I haunted the adoption board and traded off all of my January 2010 pets for more of them.
What was the first pet/item in your hoard?
I'm not sure on any of them. It's been a while since I started my hoards.