The writers and producers had filmed a number of scenes originally intended to be included somewhere in episodes 3 through 5, but these scenes were cut.
One reason for the cut was that one of the storylines showed the leader of Eden Project, Devon Adair, in a less than heroic light, and apparently someone higher up the chain of command was not happy about that. It's possible that when that storyline was cut, the other storylines were also cut because together they would no longer make a complete episode.
Months later, for whatever reason, the cut footage (including the storyline which was unfavorable towards Devon) was approved after all, so it was assembled into a complete episode called "Natural Born Grendlers" (abbreviated NBG). This episode aired after the season finale, as if the producers were saying, "Oh, the show's been canceled, so here's some extra stuff which actually happened much earlier. Consider it a bonus episode. Enjoy." NBG is officially episode 20, but the events in the episode explicitly place it very near the beginning of the season.
This leaves Earth 2 fans with a subject for discussion: where does NBG belong in the episode order? Some events within this cobbled-together episode seem to take place between episodes 3 and 4, and other events seem to take place between episodes 4 and 5.
Don't laugh; this is the kind of thing that keeps us awake at night.
Gaal uses his slimy charm to insinuate himself into the group and tries to drive a wedge between True and her father.
A horse is grown from a canister, and True becomes jealous of Ulysses. Alonzo is terrified of the Terrians and wants them out of his head badly. The group realizes that Gaal is a ruthless killer and kicks him out. The horse rides off, never to be seen again.
Ulysses is sick once again, and the Terrians ask the Edenites to free a tribe enslaved by Gaal. Alonzo is not only sympathetic towards the Terrians, but finds that he cares about them deeply. Gaal is pulled into the ground by the Terrians and is never seen again.
The Edenites discover the fate of the third evacuation pod. Danziger and Baines get deathly ill, Wentworth and Firestein die, and the group discovers the cause of the crash.
Bess begins trading with a Grendler, and she and Morgan acquire the geolock box which we see them use several episodes later. Bess resents the way Devon treats her as inferior, and finally tells her off. For a brief moment, the horse is visible on screen, but it is mostly hidden behind something and hardly noticeable. Alonzo is unable to handle life stuck on one planet with a broken leg, and can't stand the Terrians inside his head any more, so he attempts suicide, but the Terrians stop him. On the dream plane, Alonzo comes to terms with his situation, and finds peace.
Many Edenites (the majority, I think) place NBG between episodes 4 and 5. There is certainly a case to be made for this. Episodes 2, 3 and 4 form a nice trilogy with the Gaal story arc, and the end of episode 3 flows smoothly into the beginning of episode 4. Shoehorning an episode between 3 and 4 feels very awkward, whereas the gap between episodes 4 and 5 provides a much nicer place to put extra material.
Some Edenites want to make some kind of meaning out of the fact that the horse is briefly visible in NBG. But the horse came and went entirely within episode 3, so if we accept that it's the same horse, and in the same time frame, then NBG takes place somewhere in the middle of episode 3, perhaps during a commercial break. I'm not sure that works very well.
I place NBG between episodes 3 and 4. I don't like it, because I don't like breaking the story flow between episodes 3 and 4, but I dislike all the other solutions even more.
My main reason is the set of events which Alonzo experiences.
In episode 3, Alonzo basically thinks the Terrians are monsters and wants them out of his head, and there is no cut on his forehead. In NBG, Alonzo tries to kill himself and he gets a nasty cut on his forehead; the Terrians save him and show him something about life, and he finds peace and accepts them as a friend. In episode 4, Alonzo has the cut on his forehead which he received during his failed suicide attempt, and his animosity towards the Terrians has completely disappeared.
All of these things are major story components which I cannot ignore. At the beginning of NBG, Alonzo still hates the Terrians. If NBG takes place after episode 4, then Alonzo not only gets the cut on his forehead after we've already seen it, but he still hates the Terrians after he has already emotionally bonded with them in a powerful way, none of which makes sense. So, regretfully, between episodes 3 and 4 it is.
The fact that we can see the horse for a few seconds in NBG is, in comparison, trivial, in my opinion. That brief scene can easily be explained by saying that they opened a second canister, or that the horse came back briefly before dying. It's not a big deal.
There is an upside to this. Placing NBG between episodes 3 and 4 means that I get to give it episode number 3.14159265. Kudos to you if you know what that means.