Wait, so Seasons 2-5 take place within the span of a year?
Time if an iffy thing for this series. It does appear that we're in our second year, though.
Word of God is that the episodes are not necessarily in chronological order ... which is the only way it makes sense.
My Approximate Timeline:
Summer Year 1
- Nightmare Moon is Defeated (Friendship is Magic)
- Rainbow Dash masters the Sonic Rainbow
- Twilight adopts the Owl (you know its name) as her pet
Fall Year 1
- Girls go to the Grand Galloping Gala (Best Night Ever)
- Princess Luna's first public appearance since returning (Luna Eclipsed)
- Babs Seed becomes a Cutie Mark Crusader
- Discord is defeated (The Return of Harmony)
Winter Year 1
- Queen Chrysalis is defeated (The Royal Wedding)
- Princess Celestia begins taking letters from Twilight's friends (Lesson Zero)
- Heartswarming Eve Pageant
- Spike saves the Crystal Empire / Twilight's Princess Training Begins
Spring Year 2
- Winter Wrap Up
- Rainbow Dash adopts Tank as her pet
- The Cake Twins are born
- Rainbow Dash goes to the Wonderbolts Academy
- Twilight Sparkle becomes an Alicorn
- Twilight Sparkle follows Sunset Shimmer into the Mirror (Equestria Girls 1)
- Rainbow Dash becomes a Wonderbolt Reserve
- Discord is reformed
Summer Year 2
- The Equestria Games are held in the Crystal Empire
- Summer Sun Festival with Celestia, Luna, and Twilight
- Pretty much everything in Season 4 (Keys!)
- Tirek is defeated
Fall Year 2
- Twilight Sparkle goes into the Mirror World again (Rainbow Rocks)
- Babs Seed gets her Cutie Mark
- Girls go to their second Grand Galloping Gala (Make Friends but Keep Discord)
Winter Year 2
- Tank goes into Hibernation
So what, does that mean Tank isn't coming back until Season 8 or something?
I wouldn't be surprised if he didn't come back. The pets are rarely seen as it is.
On the other hand they could just apply cartoon logic to it and have him show up next episode.